Conditional formatting enables you to highlight cells with a certain color, depending on the cell’s value.
For example, highlighting positive values with green background color, and negative values with red.
(actually not only the color can be set, but every aspect of cell formatting can be applied this way - font style, size, bold, underline etc.)
Follow these links-
http://www.contextures.com/xlcondformat03.html
http://www.cpearson.com/excel/cformatting.htm
http://www.free-training-tutorial.com/conditional-formatting.html
Hope this will help you.