1. Create a variable $color = new array(); in mainfile on root level (not in a function).
2. Add the line global $color; too every theme or function you use colors.
3. Set colors in your theme like this: $color["headingbg"] = "#FFFFFF";
4. To use the color it's sometimes useful to assign it to a local variable $headingbg = $color["headingbg"]; so you can use it in quoted strings.
Hope this helps, and please consider adding it to the postnuke core, since IMHO it's simple and elegant. Perhaps not the best, but surely the best for now.