Yellow how? Is the skin yellow, fur yellow? Do you feel any foods that are orange?
I've never really had an older hedgehog's face turn yellow. I have had hedgehogs who had either food stains from carrots or sweet potatoes. And had one who's previous owner used a yellow died aspen bedding that stained the hedgehog's skin and fur. Was particularly bad on her face because she'd burrow into the bedding.
usually mine will get food and gunk all over their faces, but if the skin is yellow, and there is nothing on it I would be concerned about liver or kidney failure. Give him a bath and gently wipe his face to try and get the yellow off.
Needler likes to lay in a poop and pee wheel if he is caught running so he gets pee stains quite often on his face. Could it be possible your little one is laying in a frsh poop/pee wheel?