Raise your hand if you're absolutely mystified by the seeming effortlessness with which great designers match color. Everyone? OK, good. We're all in the same boat, then. While there is an art to matching colors well, it's not nearly as complex as it might seem from the outside.
The Color Wheel So, let's start with the basics. To the left here is a standard color wheel. First invented by Sir Isaac Newton when he observed the way light bent through a prism, the color wheel is a really simple way to visualize the array of visible light. As you can see all the colors are represented here, except for white, gray, and black. That's because they aren't really colors. Since black is the absence of color, and white is all of the colors together, they can both be combined with any other colors equally well. So, once you've visualized the array of colors in this way, you can start determining interesting ways of combining them that are pleasing to the eye and create contrast.
Monochromatic One of the most basic ways to combine colors is to simply build a palette based on colors in the same spectrum. So, like the diagram on the right shows, you could simply create a palette from various hues of yellow, and throw in a white and black to create a good range of colors.
Complimentary Another really common way of combining colors into a palette is by reaching across the proverbial aisle, and using colors that are on the opposite side of the color wheel. In this case, you could take some yellows and add in some purples for an interesting blend of cool and warm colors.
Analogous The last way designers commonly combine colors is by simply using different hues of colors along the same side of the color wheel. In the example to the right, we're combining yellows, oranges, and reds to create a fiery blend of warm colors that really add some heat (like what I did there?).
So, there you have it, folks. See? Combining colors into interesting and useful palettes isn't nearly as hard as it may seem.