The mid 2010 iMac I've picked up second hand showed what's commonly called 'yellow tinge', where the displays color temperature differs from top to bottom. Something I didn't notice upon buying (in full daylight), but pretty annoying when working with the machine in the evenings!
There is a very good website which allows you to test you screen for defects: http://imac.squeaked.com/test.php
This yellow tinge issue has been a problem for this iMac series from the beginning in 2009. If you search the net you can find -a lot- of people who back in the day brought their iMac in for repair because of this problem. According to the seller of my iMac the screen was replaced by Apple in 2012 due to pixel defects, however this replacement screen wasn't any better than the early 2009 versions!
I've read a lot about cleaning the LCD panel in this type of Mac, and how to disassemble the panel by itself. There are numerous video's on Youtube showing how this is done, and the task is pretty straightforward.
As I had two options, live with it or fix it, I went for the fix it!
Be aware, the iMac is a delicate piece of machinery. Cracking it open will expose parts that might still hold current. Don't touch the the power supply! Be sure to carefully disconnect cables as they are damaged easily, and not cheap to replace!