You have 2 different color factories represented in these 2 breeds. The brown egg laid by the PR is produced when brown pigment (produced by break down and recycling of red blood cells) is laid over a white egg as it passes through her system. Early in her laying cycle, there is plenty of pigment. So, her shells will be nice and dark, and most likely of good quality. As she nears the end of her laying cycle (late summer, into fall) her pigment supply lessens, and the stress of continuous egg production results in a lighter colored egg, and possibly decreased shell quality. After a molt and time to replenish her system, she will resume the dark shells.
In the case of the Araucana (Unless you bought her from a breeder, and she has no tail, she is not an Araucana, but is an Easter Egger) (The egg pigment is produced by break down of bile) This pigment is more likely to go all the way through the egg, and if you peel the white membranes from the inside of the shell, you should see the blue color. Varying degrees of blue (which are apt to be quite light in the case of an EE, I'm not sure how dark the blue in an Araucana or an Ameraucana depend on the specific bird. With an EE, you may see blue, or varying shades of aqua, green, or olive which indicate that she also has some brown pigment (just like a brown egg layer) to add to the shell over the blue.
The cited article spoke about several things that could affect the shell color:
Stress, disease, poor diet, treating with a coccidiostat. IMO, any of these factors, including excess heat can result in stress to the system, possibly resulting in reduction of brown or blue pigment production. Coccidiostats (at least the one I am familiar with ) work by preventing Thiamine uptake in the cocci organism. If they do so in the organism, I am wondering if they also do so in the chicken. Last I knew, Thiamine is pretty necessary. The article mentioned providing water at 50* to help with shell color. IMO, that's not WARMING the water, but it is COOLING the water. When I look at my thermometer, and see that it's been around 80* almost every day this summer, I'm guessing my birds would be pleased if their water was 50*! Again, the cooler water would decrease the physical stress on the flock. And, Aart's ? re: age of the birds: If they are nearing the end of a laying cycle, this would result in decreased pigment. Also, if they are laying bigger eggs, there may be less pigment available to cover the larger area of the egg.
Aart's ? about age of birds has direct bearing on what you will be seeing for shell quality/color. And her ? about size of coop/run also has bearing on the discussion: crowded = stressed. Parasites can cause anemia. Anemia = lack of red blood cells = lack of brown pigment. Very easy to check and treat for external parasites. Internal parasites: don't treat unless you actually know you have an issue with them. It's more normal than we realize for any animal to have a small load of internal parasites. And, a small load can actually benefit the immune system.
My suggestion: Don't jump to treating for coccidiosis or worms. You can have a fecal sample processed with your vet for not much money. If that's what you want to do. But, if the birds are all acting well, one might wonder how necessary it is to do so. Your call here, no right or wrong in it. Do your best to give them a good diet. (I am a proponent of fermented feed) Do your homework on this one. There are folks who are passionate about the benefits of FF. (I'm in that tribe) and there are also folks who are equally as passionate about not using it. Do you eat yogurt? If you buy the nutritional benefit of yogurt over milk, then the idea of FF should bear = consideration.
