The Wii U only supports Limited RGB, and playing on a Wii U misses the entire point of Retro Gaming.
So does having a TCP/IP stack and a download store :)
Retro gamers generally play on the original hardware with CRT screens. If they can't do that, they play on emulators with using controller input converters and shaders or scanline generators. Gaming on a WiiU does not recreate that experience.
Watch the videos on this channel for more info: