Question Has anyone else noticed

That rats with black coats almost always open their eyes a day or so before rats of other coat colors?
I've raised quite a few hundred litters and I've never noticed this to be true.
My litters have various colors in most of them that include blue, agouti black and Siamese and combinations of those also, and I don't really see black rats doing anything different than the other colors pertaining to when they open their eyes.