I do not think egg color can change that way.I have two Easter Eggers. One was laying lovely blue eggs and the other one lays pale green-blue. Both had stopped laying during the winter, but the blue-green layer started back up in mid Spring. The blue eggs simply stopped and I was sure that hen was going to have to be culled. I just couldn't tell the hens apart and never caught them laying. This morning I was in the chicken yard when the egg song began. It was an Easter Egger laying the first egg of the day, and I was sure it would be blue-green. IT WAS PINK. I've read what the Internet says about an Easter Egger not changing the color of her eggs..... BUT THAT'S WHAT HAPPENED! The only chicken in the coop, the only egg in the nest and it was warm while she was cackling away. She's probably been laying all Spring, but this egg will save her life if someone else can validate this phenomenon.
But it will be easy enough to check: put her in a separate pen or a dog crate or something, and wait until she lays an egg in there (or fails to lay an egg, if that's what happens.)
Regarding the two hens that were laying last year: did you ever get both egg colors on the same day? Because I wonder if one was laying blue/green eggs (slight variations in shade from one day to another) and the other was laying brown/tan/pink eggs the whole time.