My Nidoking is getting kinda up there in age, and his colors are starting to dull. Any idea what’s going on? He leads a very active team (we’re based near the exit of Kanto’s Victory Road) and is on a special diet considering his age

Actually, the answer is quite simple: he’s getting up there in age. As with humans, when certain pokémon get older, special cells that lend skin, hair, and fur their vivid pigments stop being produced, and the ones that already exist simply die off naturally. Consequently, a pokémon’s pelt may turn duller or grayer the longer they live. It’s quite natural, and there’s not much you can do about it, I’m afraid. So long as your nidoking is otherwise healthy and active, he’ll be fine.