Ok
For beginners you can change to either 3 colours.
Right click on desktop and select properties, then select
appearance tab, then select in the Colour Scheme filed,
either Luna blue, luna silver and luna green.
You can however change it to basically any colour using
Tgsoft Style XP, located from their website.
http://www.tgtsoft.com/download.html and select Style
XP from there, its shareware which means you have to pay
for it eventually.
For advanced users, use the UXTheme patcher utility. This
requires changing around both the uxtheme.dll in both the
dll cache(hidden system folder) and in system32 folder.
Advance users only.
Then go to sites like
http://www.themexp.org/ and then
you can use any theme you want and have basically any
colour you want.
-Davey