You can only have one colour scheme at a time running, and there's no keyboard shortcut for it, (although Alt+Shift+Print Screen will activate High Contrast Mode). anonomouse is right... right click the Desktop, choose Properties, go to the Appearance tab, and from the first dropdown, choose Classic Style, XP Style, or, if you have it, Royale or Zune Style.
XP Style will offer you the choices of Silver, Olive Green and Default Blue. Classic Style offers more choices, but it's not the XP style interface, and Zune and Royale styles are all glossy and shiny.