Here's the first entry for influencing egg color:
http://www.laidinbritaineggs.co.uk/Pages/color_eggshells.html
And, then this little bit of info copied from this article:
https://academic.oup.com/ps/article...shell-color-in-brown-egg-laying-hens-a-review
HOUSING SYSTEM, NUTRITION, AND EGGSHELL COLOR
With the increase in free-range commercial farming in different parts of the world and particularly in Australia, there is interest in investigation of the factors causing pale shell color and how to improve eggshell color in hens laying brown-shelled eggs. Consumer preference is shifting from quantity to quality with free-range eggs gaining popularity due to the perception that it is a natural production system (Wang et al., 2009b). Egg color is generally maintained well in cage flocks, but there is anecdotal evidence that maintenance of shell color can be more challenging in free-range flocks (Sekeroglu et al., 2010). It has been reported that eggs from free-range flocks may be lighter in color compared to those from cage systems (Samiullah et al., 2014).
Hen nutrition is an area that needs to be tested in relation to eggshell color. Hooge (2007) reported that certain probiotics improved shell color in hens laying brown-shelled eggs. Feeding
Bacillus subtilis supplemented feed to 63 wk Lohman Brown hens improved the intensity of brown shell color in the following two weeks of production. The mode of action of
Bacillus subtilis is not clear and needs further investigation; however, a possible mechanism of action could be due to amino acid residues such as His183 and Glu264 in
Bacillus subtilis ferrochelatase facilitating the insertion of metal ion into protoporphyrin (Hansson et al., 2007). Some elements, such as Fe, Cu, Mn, and Zn, function as chelating carriers at the central position of porphyrin molecules (Solomon, 1987). Feed supplemented with Fe soy proteinate significantly improved eggshell color in brown-egg laying hens (Seo et al., 2010).
Vanadium is usually found in commercial poultry feed at very low inclusion levels (Miles and Henry, 2004). Vanadium in poultry feed has a detrimental effect on shell color (Sutly et al., 2001), which can be overcome by feeding vitamin C at various levels, depending on the level of vanadium in the feed (Odabasi et al., 2006). The exact mechanism of vanadium toxicosis is not clear. The suggestion that loss of shell color in free-range laying hens could be due to elevated levels of vitamin D was tested in a study that found that vitamin D had no significant effect on shell color (Roberts et al., 2014).
GENETICS OF THE EGGSHELL COLOR
The molecular basis of brown pigment synthesis and its possible metabolic pathway in brown-egg laying hens need further exploration in order to determine the genes involved (Wang et al., 2013). The color of the eggshell is assumed to be controlled by several genes that encode proteins and enzymes, thereby regulating the production and deposition of pigment into the shell (Van Brummelen and Bissbort, 1993; Liu and Cheng, 2010), but the responsible genes in brown-egg laying hens are yet to be identified. The higher δ-aminolevulinic acid synthase activity in brown-egg laying hens compared to white-egg laying hens suggests that the trait is purely controlled by genes (Schwartz et al., 1980). Brown eggshell color is due to a dominant gene that is epistatic to the recessive white shell color gene (Punnett and Bailey, 1920); however, the phenotypic heritability calculations for dams (s2 = 0.9135) for eggshell color in brown-shelled eggs showed some dominance over sire (s2 = 0.3035) (Blow et al., 1950). Similarly, in measuring the heritability estimates of sire and dams for shell color in a Light Sussex flock, the dam heritability was higher than the sire, indicating the existence of a dominance effect (Hunton, 1962). Genetically, the quantitative trait loci (QTL) region on chromosomes 2, 4, 5, 6, and 11 influences eggshell color (Wardecka et al., 2002; Sasaki et al., 2004; Schreiweis et al., 2006). The genes coding for aminolevulinic acid synthase and aminolevulinic acid ferrochelatase are thought to be located in chromosome 1 (Schwartz et al., 1980). Based on the phenotypic observations, when white- and brown-egg laying chickens are crossed, a co-dominance effect is created with an intermediate eggshell color (Hall, 1944; Blow et al., 1950); however, when comparing the sire effects, pullets sired by the White Leghorn male produced eggs with less pigment than the pullets sired by the Rhode Island Red male, suggesting the involvement of sex-linked genes (Hall, 1944). Based on the available information, it is still unclear which genes are responsible for brown pigment synthesis in the shell gland.
The heritability of eggshell color in a brown-egg laying breed was closer to 0.50 (Francesch et al., 1997). The analysis of brown-egg laying strains and their crosses has shown a higher variance value for dam than sire (Blow et al., 1950; Hunton, 1962). A protein haplotype of ovocalyxin 32 has been shown to affect eggshell color (Fulton et al., 2012). A gene that can reduce protoporphyrin has some value in breeding for eliminating the tinted eggshell problems occurring sometimes in white-egg laying hens (Shoffner et al., 1982). Recent molecular work has established that the expression of SLCO1A2 and SLCO1C1 genes in the shell gland of a brown-egg laying hen was associated with brown eggshell color (Zheng et al., 2014). In the heritability measurements, a low genetic correlation of eggshell color with quality traits suggests that eggshell color affects other shell traits at a very low level (Zhang et al., 2005). Summarizing the genetic work conducted, it can be concluded that the genetics of the brown eggshell color in the laying hen has not been studied sufficiently to understand the process of shell pigment synthesis in the shell gland. Dunn (2011) offers the opinion that traditional methods of selection for shell color may prove more useful than molecular methods.
HEN STRAIN, AGE, AND EGGSHELL COLOR
Production of uniform dark-brown colored eggshells through selective breeding is the goal of poultry breeders of brown-egg laying hens. Significant differences in shell color have been recorded among breeds laying eggs with brown shells (Grover et al., 1980). The amount of brown and blue pigments is higher in the shell gland and eggshell of brown and blue eggs when compared to white-shelled eggs, which indicates that pigment is breed specific (Liu et al., 2010). Hens that lay lighter-colored eggs at the start of lay also lay lighter-colored eggs in the late laying period (Odabasi et al., 2007).
Generally, the eggshell gets paler as the hen ages (Odabasi et al., 2007). It is not clear how this happens, but increase in egg size with hen age is considered one of the main factors (Hunton, 1962; Grover et al., 1980; Odabasi et al., 2007). In a longitudinal study of the effect of hen age on brown eggshell color, there was no significant difference between the eggshell color at 35 to 75 wk, but the 25 wk eggshell color was significantly darker than all other age groups (Samiullah et al., 2014). Following the same flock at different ages (longitudinal study) or different ages of different brown-egg laying flocks (horizontal study), eggshell color gets lighter as the flocks get older (Samiullah, 2012). The amount of protoporphyrin IX in 1 g of whole eggshell in 33, 50, and 67 wk old HyLine Brown flocks was not significantly different (Samiullah and Roberts, 2013); however, the amount of protoporphyrin IX measured in the cuticle itself was significantly higher at 50 wk compared with 33 and 67 wk eggs, which suggests that shell color gets lighter with flock age, and the amount of cuticle present on eggshell surface has positive influence on pigment present in the shell. It can be concluded that eggshell color generally gets lighter as the hen ages.
INFECTIOUS BRONCHITIS VIRUS AND EGGSHELL COLOR
Infectious bronchitis virus (IBV) is a coronavirus of economic importance to the poultry industry around the world. IBV can infect chickens of all ages and has the capability to multiply in various epithelial tissues, including trachea, lungs, kidney, ovaries, and oviduct (Ignjatovic et al., 2002). The incubation period of IBV in fully susceptible hens is about 18 to 36 h (Sevoian and Levine, 1957). Many IBV strains affect egg production and cause paleness of eggshell color in brown-egg laying hens (Chousalkar and Roberts, 2007). Previous studies show that IBV strains differentially affect the epithelial tissues in the shell-forming regions of the oviduct in brown-egg laying hens (Chousalkar et al., 2007). The Australian IBV strains are mainly respiratory and nephropathogenic with variation in pathogenicity, but they are also capable of infecting the oviduct (Ignjatovic et al., 2002). There was no significant difference in the histopathology induced by two strains of IBV (T and N1/88) in the oviduct of vaccinated and unvaccinated birds (Chousalkar et al., 2007). Comparing the histopathological severity of the different strains in the oviduct, T strain was more virulent followed by N1/88 and then the vaccine strains, Vic S and A3. All strains caused paler coloration of brown-shelled eggs (Chousalkar and Roberts, 2009; Chousalkar et al., 2009). The mechanism of action of IBV strains on pigment synthesis is not clear, but pathology induced in the oviduct by IBV may disrupt the cellular mechanisms responsible for the secretion and subsequent deposition of pigment onto the eggshell. Further research is needed to explore the mechanism of action of IBV on pigment secretion and deposition. IBV strains have been shown to cause disorder of eggshell formation by disrupting isthmus gene expression of collagen type I and calcium binding protein 28kDa (CaBP-D28K) in the uterus (Nii et al., 2014). The viral antigen localizes only in the epithelial cells lining the oviduct (Crinion and Hofstad, 1972). Different strains differentially reduce the beating of epithelial cilia (Raj and Jones, 1996). Other diseases that cause deterioration of brown shell color are Mycoplasma spp. (Gole et al., 2012), Newcastle disease, and egg drop syndrome 76 (Higashihara et al., 1987), but their mechanism of action is also not known. Some parasitic diseases (e.g.
Leucocytozoon caulleryi) that cause damage to the oviduct (Nakamura et al., 2001) may result in the production of paler-shelled eggs.
STRESS AND EGGSHELL COLOR
One of the major problems in the assessment of poultry welfare is measurement of stress. Handling and relocation stress for about 4.5 h prior to subsequent oviposition has been shown to delay hen oviposition time by about 3.0 h (Reynard and Savory, 1999). Hens become unable to lay if the level and duration of stress exceed a certain level (Reynard and Savory, 1999). Molting stress has been shown to severely affect eggshell color once the hen starts laying again, with the extent of the response being different among individual hens (Aygun, 2013). Hens kept at high densities in cages are more stressed compared to hens in individual cages or those at low stocking densities (Mills et al., 1987). Stress factors, such as cage design, high cage density, fear, and frequent disturbance can cause brown-egg laying hens to lay lighter-colored eggshells (Walker and Hughes, 1998). Shell abnormalities such as pale coloration or extraneous calcium may be caused by exposure of hens to environmental disturbances (Hughes et al., 1986), larger group size (Mills et al., 1987), certain drug infusions (e.g. adrenaline, nicarbazin, sulphonamides), and management failure. If such disturbances occur shortly prior to oviposition, the egg is retained in the shell gland and extra calcium coating takes place, which masks the brown shell color (Hughes et al., 1986). Physical stress, such as experimental removal of feathers and high environmental temperature, has a detrimental effect on pigment deposition onto the shell (Tangkere et al., 2001). An intrauterine injection of prostaglandin F2α can cause the secretion of pigment and induce quick oviposition with paler shells (Soh and Koga, 1999). A drug, indomethacin, administered by intrauterine injection, has been shown to completely inhibit the secretion of pigment in the shell gland (Soh and Koga, 1999).
NICARBAZIN AND EGGSHELL COLOR
The coccidiostat drug nicarbazin, when fed to poultry, resulted in depigmented eggs at various levels within days of its administration (Hughes et al., 1991). The severity and duration of the effect depend on the drug concentration and duration of treatment (Schwartz et al., 1975; Hughes et al., 1991). This drug did not affect porphyrin synthesis in oviduct tissue (Polin, 1959; Schwartz et al., 1975), but how it affects deposition of protoporphyrin into the eggshell is not clear. Similarly, it did not prevent the elevation of erythrocyte protoporphyrin levels in the regenerative phase, following hen bleeding nor did it affect the formation of porphyrin from δ-aminolevulinic acid incubated with homogenates of uterus (Polin, 1959). The effect of the drug is reversible and shell color is restored usually within 6 to 8 days, once the drug is withdrawn (Hughes et al., 1991).
In an
in vitro study, nicarbazin did not prevent tissue porphyrin synthesis from aminolevulinic acid, as tissues from hens fed nicarbazin formed the same level of porphyrin as the control group (Polin, 1959). These findings suggest that there could be another pathway for porphyrin synthesis other than the glycine succinate cycle. The decrease in shell pigment deposition in nicarbazin fed laying hens varied with the amount of nicarbazin-fed (Polin, 1959).
SHELL COLOR AND EGG QUALITY
Eggshell and egg internal quality are influenced by various factors such as egg weight, shell weight, specific gravity, shell breaking strength, shell deformation, shell thickness, albumen height, and yolk color. A significant correlation between brown shell color and shell strength (Yang et al., 2009) may indicate that brown eggshell pigment affects shell quality. A dark brown eggshell color has been linked to higher eggshell specific gravity, which is a shell quality indicator (Joseph et al., 1999). Brown eggshell color has been positively correlated with some shell characteristics such as shell strength and hatchability (Sekeroglu and Duman, 2011), while egg internal quality has no correlation with shell color (Yang et al., 2009). Further, it has been suggested that some shell quality parameters such as shell strength, shell weight, shell thickness, and shell ultrastructure can be assessed via shell color because of significant correlations between the shell quality indicator and shell color (Schreiweis et al., 2006; Yang et al., 2009); however, others have provided conflicting evidence (Joseph et al., 1999; Richards and Deeming, 2001), and thus shell color cannot be applied reliably as a quality assessment tool.
Has anyone experimented with different types of treats that have impacted the color of their eggs (the shells, not the yolks, we already have super vibrant, almost neon orange yolks)? My girls are free range and they all get treats together, so I figure if the blackberries do impact their shells, my green layer might produce more aqua or turquoise colored eggs. I'm good with that! This is what I get as of now 
And thank you Penny/LittleRedFace! This is what I picked up from the nesting boxes yesterday. We get around 6 eggs a day with 7 layers, my bowl full of eggs was a collection of a few days for a "photoshoot" 
Hopefully chicken math doesn't kick in bc we've got a line of people waiting for eggs as it is. 
