Web Development - Key 6

6. Avoid bad color combinations

You want to have good contrast between backgrounds and content so your design is easy to read in browsers. When you use a dark grey text color with a light grey background things will be difficult to read. Just as yellow and black send signals of being somewhat "discount". Red text color on a blue background will make text appear as though it's jumping around.

You will no doubt appreciate checking out this site: http://www.colorschemer.com/online.html to get a better understanding of design colors.