Color is everywhere and conveys a message even if we don't realize it. While this message can vary by culture it pays to know what colors "say" in your own corner of the universe, and even what color means to your target market.

If you don't think that color speaks just complete this sentence, "red means ---- and green means -" even a child will know what red means stop and green means go. If such simple ideas work for all of a given culture or market what could it mean to the graphic design of your website, brochure, or product if you know some of this information.

First let's start with the basics. The color wheel. We've all seen it. The color wheel shows the basic colors, each wheel is different in how many shades of each color is shown, but they are essentially the same.

Color harmony, colors that go together well. These will be colors that are next door to each other on the color wheel. Such as blue and green. In reference to clothes these colors match each other. Instinctively most of us know which colors go together when we dress ourselves every morning.
Color complements, colors that set each other off, they complement each other. These are colors that are opposite on the color wheel. Such as blue and orange.

Color depth, colors can recede or jump forward. Remember that some colors seem to fall back such as blue, black, dark green, and brown. Other colors will seem to step forward such as white, yellow, red, and orange. This is why if you have a bright orange background it may seem to fight with any text or images that you place on it. The orange will always seem to move forward.

Now you have the basics so let's go further. Just because to colors go together or complement each other doesn't mean that yo necessarily want to use them on your project. I opened this article with the meaning of colors now here is an example, keep in mind this is one example from western culture.
Color Survey: what respondents said colors mean to them.

Happy = Yellow Inexpensive = Brown
Pure = White Powerful = Red (tomato)
Good Luck = green Dependable = Blue
Good tasting = Red (tomato) High Quality = Black
Dignity = Purple Nausea = Green
Technology = Silver Deity = White
Sexiness = Red (tomato) Bad Luck = Black
Mourning = Black Favorite color = Blue
Expensive = Gold Least favorite color = Orange

So in designing your project it's important to know what colors mean. You can now see why a black back ground with green type would be bad, beyond being nearly impossible to read, if your target market thinks that black represents mourning and green makes them sick. There are exceptions to every rule of course.
So you may want to include some research in what colors mean to your target market. Colors that would get the attention of a teen would probably annoy an older person and the colors that appeal to the older person wouldn't get a second look from a young person.
Color may be one of the most overlooked aspects of design.