How can I change the colors of my forums using vBulletin 4 ?

Here's a link that details all the styles you need to edit to modify your skin / template in vBulletin 4:

http://www.vbulletin.com/forum/entry.php ?2373-vB4-Skinning-Your-Forum-in-10-Easy-Steps

You need to go into Styles & Templates -> Style Vars to edit most of these.