Great photos are all about light, shape, form, contrast and... color. Color is powerful. It can create mood, passion and tell a story. This example from Naveen Jamal's Flickr Stream is a good example.

If you're trying to get a blue sky, don't be afraid to underexpose some to avoid losing the rich colors. That's what I did in this photo from my own Flickr stream. You can also use a polarizing filter which will help bring out a deep blue color from the sky and control the way light is reflected off of flat surfaces in your image.

Look for the chance to combine color with texture and pattern. This photo from 4blueeyes Flickr stream is a great example of color AND texture.

Flickr has a whole section devoted to different color groups. One interesting group is the complementary colors group. This photo of M&Ms, from Sabinche's Flick stream has green and red, two colors that are opposite each other on the color wheel. You can read all about color wheels and color theory in this Wikipedia article.