My TV is in OTP box mode which looks great for almost every app on Roku, expect for Youtube. In this mode, the black level is limited, meaning 16-235, vs full 0-255 RGB. As almost all content is made in limited mode, this works perfectly for all the channels - netflix, prime, hulu, etc.
However, Youtube looks really washed out, meaning the blacks come out as grays and the whites are too luminescent. I have the ability to use either my TVs own youtube app or my FireTV 4K Stick youtube app. Both those versions handle the colors properly. So my guess is that Youtube/google has miscoded this particular app on this particular platform. I've reached out to them and it they never followed up on it. This needs some pressure from the roku developing team to correct.
If you too experience poor color handling with your YouTube app, please respond and let the developers get to work!