On the web we work in a color space called RGB (red, green, blue). RGB is an additive color system seen by projected light. When all of the colors are on our screen at once we see white. You ADD colors to get white.
CMYK it is a subtractive color system seen by reflected light and therefore used for printing. You SUBTRACT colors to get white.
When we reproduce these colors for online viewing we use a hexidecimal code to tell the computer how to translate the color. It is a 6 character string -mouse over the swatches at right to see their hexadecimal codes. Each grouping of two characters stands for a value of either red, green, or blue.
Using combinations of shades and tints of these colors is one way that a designer may choose a color scheme.
Using these colors in different proportions will give a different mood to your site.
Remember that there is a psychology to color. People associate different moods with different colors. If you were building a site for tropical travel destinations you would probably want to use warm colors to give a sunny warm atmosphere and provide your clients with the feeling they are looking for in the subject matter you are presenting. Use color psychology to your advantage. It is not very complex but rather quite intuitive.
Remember how large type font can be compared to visual screaming? Well the same comparison can be made when using bright, highly saturated color. Generally it is best to use subtle coloring on your site except for in areas of emphasis. Another good rule to remember is that the best color combination for reading is black text on a white background.