If you don't want to use mzScript you can achieve your goal by defining a variable for each color scheme.
BlueScheme true
RedScheme false
YellowScheme false
GreenScheme false
IF BlueScheme
BackgroundColor 000000
ENDIF
IF RedScheme
BackgroundColor 0a0301
ENDIF
IF YellowScheme
BackgroundColor 4a4341
ENDIF
IF GreenScheme
BackgroundColor 00f100
ENDIF
If you only have 2 color schemes it's even easier...
BlueScheme true
RedScheme false
IF BlueScheme
BackgroundColor 000000
ELSE
BackgroundColor 0a0301
ENDIF
...of course I'd recommend using mzScript anyway as once you get your head wrapped around how to use it, suddenly you have SOOOOOO many more things you can do with your theme :)