Normally, I would have said that the yellow one was a golden albino, but it's clearly not based on the eyes. The other is almost certainly a leucistic. It may be that the golden one is as well, but with an unusually high concentrations of something that affects the yellow color. If it is, in fact, due to the food, I would have expected that the other axolotl to have it is as well, so likely it's genetic. I do know that, for golden albinos, the yellow does build up as they get older, but again, this isn't an albino. Frankly I'm a bit confused as well, sorry I couldn't be of more help.