Right-click the Diablo 2 shortcut, go to the compatibility tab and check "disable desktop composition". You may also have to run it in XP mode, but try without first.
Oh and also, when I run Starcraft the colors are also messed up as you describe unless I first open up the "Personalize" screen. Right click the desktop, then click Personalize. Then try opening D2 while that screen is open. For some insane reason this fixes the Starcraft problem in Windows 7 so it might help you even though you're using Vista.