I would proceed cautiously when feeding "colored" eggs shells to your chickens - wether or not they are safe is an individual opinion. Personally, I have seen some data linking artificial food colorings/dyes to ADD/ADHD as well as other Diseases - if there is a concern in human consumption, I would imagine that the dyes would be more concentrated in the chickens much-much smaller bodies - then passed forward into their eggs (as would be passed from a human mom to a baby in the womb) it's a little late for this year, but here is a great article with natural egg dye options