So I assumed that the Wii U would output Wii games in the RGB color space.
It appears I was wrong.
When I am in Wii mode, my TV doesn't have access to its RGB settings. Which only normally happens when I'm playing a Bluray in Y′CbCr. So even over HDMI, the Wii U's Wii mode is still using the same color space output as the original Wii used for component video, just in digital form. Weird.
Oh wow, now that's quite interesting.