Regarding the purple sheen...... scroll down to bolded content.
from
http://sellers.kippenjungle.nl/page2.html#t12
Genetics of feather color:
The genetics of feather color and patterns is an active topic of poultry science research. Much of the work that was done prior to the late 1980s is now considered out of date. Because a number of genes interact to determine feather colors and patterns, it might seem to be too involved for the average enthusiast. I dont believe that this is the case, however, the topic of feather color and patterns may be beyond the interest and motivation of some enthusiasts.
White is actually all the colors combined and black is the lack of reflection of light in the visible range, so one might argue that black and white are not really colors technically. However, if we count black and white as colors, chickens have only three basic colors: black, white and red (gold).
The colors of chickens are achieved by diluting and enhancing or masking black and red (gold). For example, Rhode Island Reds have the gold gene with the dominant mahogany (red enhancing) gene. A blue chicken is a black bird that has the blue gene which dilutes black. Two copies of the blue gene give a splash effect. A white chicken can be achieved in a number of ways by inhibiting black and red pigmentation with combinations of genes (dominant white, recessive white, silver, Columbian, Cuckoo barring).
Some perceived colors of feathers are due to the structure of the feather and not any pigmentation. The purple and the beetle green sheen that can be seen in some poultry is due to the way the feather structure reflects light rather than the presence of a pigment.
First, we need to define a couple of terms. In poultry there are primary and secondary color patterns. Perhaps it is better to define secondary patterns first. A secondary pattern is a pattern that appears on individual feathers. These are patterns like single and double lace, mottle, and so on. Primary patterns are color patterns that involve the entire body of the bird. An example is the silver Columbian pattern. In the Columbian bird, black is restricted to the hackles, wing bow and tail. The silver Columbian is a white bird with some black in the neck, wing and tail areas. Because this pattern is not manifest on individual feathers, it is a primary pattern.
To construct a chicken having a particular color scheme, one begins with the background or the E-locus gene(s). The other color and (secondary) pattern genes essentially modify this background. Please refer to the table at the end and the pattern table below to see the choices and comments (other E-genes have been proposed but they are not yet well accepted). Some of these are: E, extended black or nigrum; ER, birchen; eWh, dominant wheaten; e+, wild type; brown, eb; speckled, es; buttercup, ebc; and ey, recessive wheaten. These genes cause recognizable chick down color and influence the adult feather color, sometimes male and female feather colors are influenced differently. For photographs of chicks with an assortment of E-genes the interested reader is directed to Poultry Breeding and Genetics, R.D. Crawford, ed., Elsevier, 1990 pages 115-117.
As an elementary exercise, lets build a white chicken. We can start with wild-type background, e+, and require our bird to have two copies of this gene. We can suppress the red in the chicken by adding the silver gene, S, which has the effect of changing red to white. Black is suppressed (changed to white) by the dominant white gene, I, however this gene is leaky (see the table for comments) and allows black specks through. A good helper gene in this situation is the Columbian gene, Co, since it is a restrictor of black. Although this set of genes is not the only set that will yield a white chicken, it is one of the ways a white chicken can be obtained.
Please see the table of chicken genes (Part III) for more comments about plumage color genes.