YouTube is almost exclusively HTML5 video at this point, not Flash.
That said, the symptom is the same for both technologies. Green Screen usually means that video decoding failed. Typically this is because you have old/buggy graphics drivers. If your drivers are current, then disabling hardware acceleration in your browser will allow you to work around the problem until updated graphics drivers become available for your hardware.
The tech support pages for your browser of choice should show you how to disable hardware acceleration in your browser.