This usually points to LCD screen damage or deterioration rather than a simple setting issue. Rainbow-like patterns and edge discoloration often mean the display itself is failing. Based on the reported symptoms, the practical fix is typically LCD replacement.

It may continue working for a while, but the screen is likely to get worse over time. If the camera still takes photos normally, you can keep using it as long as the display remains usable.

For next steps, contact Canon or a repair shop for an estimate. If the repair cost is high relative to the age and value of the camera, it may make more sense to keep using it until it becomes unusable and then replace the camera.