Display doing odd things
My 27" iMac with El Capitan OSX10.11.4 displayed flickering white screen, blue screen, then solid white after downloading Adobe Flash Player from CNN News (I download Flash in order to play a news video.)
When I restarted, the iMac would display the Apple for a short while, then startup would stop, and it would show a vertical green stripes over the white screen. Or sometimes a blue screen then a static white screen
I spent 2 days reading tech threads trying to diagnose the problem (had no idea what caused it at first.) I could boot the iMac after resetting SMC and PRAM, everything seemed fine again, but seems any video I played would cause the iMac to crash (a flickering white screen, then solid blue, then solid white.) So, I suggest you disable Wi-Fi while completing the following fix, use a separate computer for internet research.
To boot/access my iMac I followed, in order, these standard Apple steps:
Unplug both ends of power cord. Unplug any peripherals. Wait 1 minute. Plug in power cord and restart.
Start Mac, hold Command, Option, and P and R Keys until Apple appears. Restart.
Safe Mode Boot:
Start Mac, Hold Command and R keys after chime and until apple shows up. Then proceed to Disc Utility and run First Aid.
On a separate computer, I found the incredibly helpful thread (link below) and downloaded the Adobe Flash uninstaller to a thumb-drive. I uninstalled the Flash Player from the iMac.
I downloaded (to a thumb drive) the official Flash Player version from Adobe, installed it on my iMac. The flashing and blue-then-white screen problem have completely disappeared.
Is this a good question?