Colour theory goes right back to Goethe’s colour wheel – basically opposite colours on the wheel tend to go well together.  However, there is an even more basic rule: try not to add more than 2 fundamental colours to your banner designs.  And of course, these colours must compliment each other.  Introducing more colours runs the risk of making your banner “noisy” which web surfers have learnt to totally tune out of. Of course there may be exceptions to this rule, but try to tone down colour use.

If you know the specific web page your ad will be on, try to match the ad banner colours to the page’s colour scheme.