This is perfectly normal. I have had 2 go through that change. I bough them at WalMart and they were normal colored
CAE's less than an inch long. Last summer (2007) I bought two - one stayed normal colored and the other got yellow splotches. It didn't go fully yellow, but it had yellow splotches. Those two went to a friend for her school tanks. This last summer I bought a couple more, and once again, one of them turned yellow. It turned solid lemony yellow. Now it is gettinggreen scales on its back half again. I don't know for sure if it is a difference in the water temperature, water parameters, or if it is just a normal color mutation, but it is kinda cool to have a chameleon fish! Here are a couple of pics so you can see what mine turned into. The first pic is the two from last year, and the second pic is the all yellow one about a month before I brought it in for the winter. Hard to believe that every one of these fish started out with identical coloring!