Quit a lot of videos that are viewed inside of browsers try to use the vp9 codec, and by the look of things it is not yet fully supported (both in chrome and the android /framework itself).
I have spent hours of my time trying to figure this out, simply because it was just bugging me. I could have easily just watched the video in the YouTube app itself which seems to work just fine.
This fix is assuming that you use the chrome browser on your device.
Open up the browser and type in to the address bar chrome://flags
Next, find the option "Disable media source API" and press enable so it now says disable.
Now find the option for "Enable GPU rasterization" and set it to "Force-enabled for all layers"
That's it, now you can watch and enjoy your videos without the green/purple tint overlaying them.