If the gills have changes colour then it could be a sign of ammonia burn, but ive got a white platy with bright red gills.... Some of them are just coloured that way.
Oh. It may just be his coloring, as I just looked at earlier pictures of him and he had the magenta color.. but less of it before. I think it would be good if I dipped him in a salt bath.... just to be sure... since my red platys had symptoms of scratching themselves on decorations near the gill area right before they died, it might be gill flukes. If only I didn't have school tomorrow, I could stay home to watch my fish all day..... ah.