Solid yellow flashing background on iMac 27

Hi, I was just looking at some pics in Photos on my 27 inch iMac and all of a sudden the screen behind the window wenta solid bright yellow. This then broke up to solid yellow flashing blocks, what's happening? Had to force quit all apps then did shut down and restarted. Seems ok now but worried that there is a problem. It's a late 2015 model, about 2.5 years old. Running Mojave 10.14.6.

Hopefully just a one off quirk but would like some reassurance, use it every day for work so worried.