Chosen Solution
After several years of flawless functioning my iMac suddenly comes out with a coloured herringbone background. The thing is working as usual, but it’s very annoying, and I cannot judge colours anymore
Do you see the pattern and colours when you first start up the computer too, or just while you are in OSX. If you see it all the time, then it sounds like your iMac could be part of a large number of the G5 iMacs that have fallen prone to graphics card failures. These models are notoriously know for having issues with either their capacitors or their video cards. If the issue lies with you graphics card, then the only way to fix it is to replace the logic board. Some people have had success with doing what they call a re-flow of the graphics card, but I would only suggest this is you feel comfortable with taking your mac apart, and at the same time have nothing else to loose. I had 3 of the original G5 iMacs in my household and they have all fallen to graphics card failures. You can probably find a lightly used logic board on eBay if you are really wanting to salvage this mac, most of these older parts are cheap of you look in the right places. Hope this Helps
Connect to an external monitor. If you see the same thing on it, you have a video card/logic board issue. If you don’t (display is normal) you have an internal display chain: LCD or Cable problem) If this Answer is helpful please remember to return and mark it Accepted.
