For the nonartistic, color theory is hard to grasp. Each color, hue, shade, and tone has its own complementing and clashing colors. Put complementary colors with each other, and you have a pleasantly fashionable apartment/ensemble/desktop scheme. Put clashing colors together, and people will think you're colorblind.
Now you can keep all those different colors together on your iPod. Gizmodo reported on the Koloroo KolorWheel, an iPod widget that turns your iPod into a color wheel. Spinning the iPod Click Wheel cycles through the color wheel, so you can see exactly how any shade of any color looks against any other color.
Unless you're an artist or a designer, this really isn't the most useful thing you can put on your iPod. On the other hand, it's full of pretty colors and only costs $8, so it could still be a cool little app to play with.