Eggs from the same bird can vary in colour a bit - how dramatic is the change that you have seen - pure white to dark brown, or just a darker shade? A small change in egg colour can be perfectly normal - I have girls who lay eggs in shades from light cream to light brown, but I can usually identify them from the size, shape or texture.
Can you identify who is laying which eggs? If you can check on the girls regularly (even if it is only on the weekends, if you are at work all day in the week), you can usually see who is on the nest, and collect the egg immediately she comes off, so you will begin to recognise what colour/size/shape egg each bird lays. (And there can be very identifiable differences between birds). My girls have names, so when I collect an egg that I know was laid by a specific bird (90% of the time), I mark it with her initial - that way I can see any changes that occur, and spot any potential problems.
