This issue is raised in the latest
MacFormat, so I thought you might be interested in my recent experience. I purchased my first iMac in April, ergo I am an inexperienced user, and immediately experienced what is apparently called a rainbow screen on shutdown. Like you, I suspected a graphics card fault but I was assured all was okay. It was suggested that it was a software problem and I spent considerable time and effort making sure that there were no corruptions. Also, I tried all the potential solutions in your Genius Tips article without success.
Still suspecting a software problem I identified that the rainbow was caused by either the remnants of an Excel spreadsheet or an open notification when closing down. I solved the problem short term by always using ‘force quit’ before closing down. I then discovered that using the + and keyboard shortcuts to close programs solved the Excel problem. The other part was solved by always making sure that any notifications were dismissed. Another solution I found was to purge memory before shutdown.
There is much discussion about this on the internet and a lot of blame being placed at the door of Apple, but nobody else seems to have found the solution I now use, or indeed any simple solution. I suspect my long history from MS-DOS onwards, where correct process was drummed in to me, has led me in this direction. Many of us are used to the foibles and relative instability of Microsoft Op Systems that requires user knowledge of the nuts and bolts to resolve. The Apple Unix-based product seems to me to be aimed at people who want to use the computer without knowing what is going on under the hood. Also, I have now adopted the
+ shortcut to close down. It seems to be much more effective at cleaning the system, and apparently purging memory, on close down. by Norman Jackson
We’d always try Howard’s advice first, which is to reset the SMC and NVRAM. See issue 316 or head to bit.ly/resetSMC_VRAM. If that doesn’t work then readers will benefit from your other solutions Norman.