The wii u's backwards compatibility with wii games has input lag and color distortion in my eyes. I may get a og wii. No issues when on the wii u side.
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I have a Wii, and my vWii looks even better than my Wii honestly.
I've had no real problem with it, try asking the homebrew community on discord(dot)gg/nintendohomebrew.
But when comparing to component (wii) and vwii (hdmi) I like the wii more
don't worry - this isn't just your eyes! Nintendo made real errors with color and video output. some of it stems from the Wii U itself but more of it is prevalent in Wii Mode. check out evWii and Wii VC Launch!
you may also be interested in making your Wii U video output HDMI-to-VGA-to-HDMI with the help of some adapters. the Wii U only outputs at limited range RGB (16-235), but some adapters will force it to full range (0-255)
if your TV expects one and you give it the other, things might look either washed out or too saturated!