Tinted protein filler is a miracle product. You can get it for cheap on Amazon or at your local BSS.
Protein filler goes into the hair shaft and fortifies the hair to help keep it from breaking and becoming brittle. The tinted kind of protein filler goes one step further. It has mild tinting agents in it that can gently replace color that may have washed or otherwise faded out of your human hair wigs! This is WAY gentler than dying the wig again -- and it is actually good for your hair!
If the color is really faded, put it directly on the wig where needed, gently work into a foam, and rinse out.
If you're trying to prevent loss of color, just add it to your conditioner and rinse out as normal! :)