Found that the ‘latest’ (11.2.x) Adobe Flash Plugin for Linux (x86_64) makes YouTube videos look like something from a bad trip..    Tried different browsers, but the fault was still there..    Videos from news sites (BBC etc.) seemed to be OK..

YouTube – Purple

So – reverted to 11.1 version of Flash Plugin (from Adobe archives) and all is well, again..

YouTube purple – fixed

As the 11.2 version is supposed to be the ‘last Flash for Linux’, I hope they fix this soon!

https://bugbase.adobe.com/index.cfm?event=bug&id=3164063