I asked about the water parameters as skin discolorations are often caused by water (ammonia) poisioning. When that wasn't the case I wasn't afraid to admit that was the extent of my knowledge in my area. If you have a problem with that, too bad. I offered my suggestion that it might be a natural color change. Angelfish do go through some skin color changes, there was even a thread about it recently. Without a pic it would be difficult to hazard a guess.
Take the time to read my posts, I have offered answers on more than water parameters, on many occasions, but water quality is the cause of a lot of problems in fish.