Super Nintendo Yellowing
The console on the right is the color your SNES is supposed to be. The console on the left is the color your SNES probably is. Originally, both consoles were the same color, but over time, the main section of the top half of the shell of many Super Nintendos turned from light gray to that sickly coffee and tobacco stained teeth yellow color. Everything else stayed the same. It's clearly not exposure to the sun, because the discoloration is uniform. (Additionally, my SNES was never exposed to direct sunlight for long periods of time.) What is it, then? Thankfully, someone out on the Internet did the research for us and wrote an article about it: www.vintagecomputing.com/index.php/archives/189 (I salute their thorough nerdiness!)
Basically, the plastic in your SNES case is, in a way, rusting.