I had molly fry about a month and a half ago. there were 19 that i had saved. as fry, they didnt have much color. but there was a difference in what they looked like. There were some that were solid white, some that were brown, and some that looked like they had a little blue in them. It wasnt something that stayed with them long. at this point they are all white, like the parents. As for it being yellow, i guess it would depend on what is in the tank. I wouldnt say that it isnt possible, because that is one of the things i have learned with fish keeping, that some things just happen, no explaination as of yet can really explain it. I was also thinking that if your cousin had added a new fish or plant to the aquarium, it may have been a hitchhicker baby. ive heard of that before.
Another thing, i have platy also, and i saved 5 out of the babies i got from platy. same mother, which was a sunburst platy. She was pregnant when i got her, so i dont know what the father was. But out of the 5 babies i have, 3 are mickeymouse, and two... well they are bluish green, their tails are beginning to turn orange which i imagine will then go to red. but i think they are also sunburst platy. And i dont know if this is right or not, but two of the mickey mouse platy babies... might also be pot bellied... ????? So as i said before... i wouldnt rule out anything unless someone has the facts to back it up.
Good luck with the babies.
Janell