Maybe it's your HDMI cable? If your cables are all plugged in and stuff, try changing the plugs back to how they were on the Wii but put it in the Wii U.
I am using HDMI and I secured everything and checked again. I also swapped out the HDMI with the one on my PS4 which works fine and still had the same issue.
Ill throw on the pictures showing the Wii U part is working fine and just the Wii part is giving me the color issues.





