I had a phone with one on the back and currently have one with it on the screen. Personally I had absolutely no issues with its functionality in the screen and do prefer it over the back. I like that I can unlock my phone without having to pick it up off the desk or while its still in the mount on my vehicle dash.
Seems to work perfectly fine to me but maybe it’s a quality issues with the brand of phones. Mine is a OnePlus and it’s almost 4 years old now, still works fine and never had any issues.
I mean, can you really say that something doesn’t have a colour if you see it as that colour? Does it really matter that the colour is coming from different refractive indexes rather than that wavelength not being absorbed by the material? If it consistently happens in that set of circumstances, isn’t it fair to say that is its colour? I.e. the coating, oil slicks