iMac 21.5-inch (2015) coloured screen edges

Is there a reason why you're showing the issue using a gray background? A solid gray pattern like that would highlight variances in uniformity on any display. Try displaying the same type of gray background on your TV and see how it appears. I did that on my 55 inch TV and now I can't unsee the uniformity issues. Fortunately though, the uniformity variations on my TV don't affect the quality of the image during normal viewing.

Since you indicated your system is not covered under warranty you need to determine whether the issues you highlighted are affecting your ability to use the iMac, or whether they are only visible because you are displaying a worst case test image. If you are simply making an observation, and the uniformity issues do not affect the usability, then you may want to continue using the iMac as is. If the variations affect your usability (i.e. full-screen photo retouching) then you may want to consider paying to replace the display.

In the unlikely event that the variations you are seeing are the result of image persistence then the following article may be helpful. Unfortunately the images showing the issue aren't detailed enough for me to clearly see whether image persistence could be a factor. As dialabrain suggested, you can always take it into a service provider for an evaluation.

Avoiding image persistence on Apple displays - Apple Support

You can still take it to an Apple Store for a free diagnosis.