I'm not a superb artist, but I've learned a little bit about color and light theory in my attempts to improve my artistic skills. What I'm seeing in a lot of games is a tendency to use pure green for grass, pure blue for water, etc. It's a natural assumption to make, but it's not quite the right one.

The picture on the right is a screenshot of Chrono Trigger. The odd-looking green square toward the center of the image is a region where I filtered out all of the red and blue components of the color. Notice how the rest of grass in the image looks a lot more natural than the bright green patch.

I encourage you to grab a screenshot from your favorite game (2D or 3D -- this applies to both), load it up in your pixel editor of choice (I'm using Gimp at the moment), and poke around with the eyedropper tool. If you examine the grass colors, you'll notice that they all have a red and a blue component, as opposed to just green.

Furthermore, you'll discover that brighter colors are more yellow and darker colors are more blue. This is because of the way that natural sunlight works. Sunlight is yellowish, and as such, anything in direct sunlight will pick up a bit of that yellow. On the other hand, the rest of the sky is blue. Anything in a shadow will be lit mostly by the sky, and will pick up a bluish tint.

When using this, remember that subtlety is the key. Don't make your brights too yellow or your darks too blue. Adjust them carefully until they look natural.