Here\'s my setup
The Mac OSX Terminal.app in Snow Leopard does not support 256 colors, which is required for the ir_black theme (this is the theme I use).
Download and try something like iTerm.app (http://iterm.sourceforge.net/), and you shouldn't have a problem with colors.
Or you could use MacVim (http://code.google.com/p/macvim/)
Edit: As of OSX 10.7 Lion, the built in Terminal.app now supports 256 colors. See the comment below by Chris Page for how to achieve this.