Why does my display have a yellow hue?

The display on my 21.5” mid 2010 iMac has a yellow hue that is darker toward the bottom of the screen and fades more true to color at the top. I've been looking everywhere for suggestions as to what it could be and think it may just be the backlight for the display. Is this something that happens early on before a backlight goes completely bad? Is it an affordable fix? I just bought it refurbished from macofalltrades.com. I really wouldn't even mind the way it looks, but I use Photoshop regularly and its a concern because the inaccurate color on my display effects the overall quality of my edits. Any suggestions?