For the most part, I use TkDiff as the Git difftool when working under X Windows. TortoiseGit also has a nice GUI diff tool for when I do occassional work on Windows.
However, when working in a Linux "terminal" environment, your Git difftool choices seem to be limited to vimdiff. This is OK, but most of the color schemes under vimdiff are terrible.
I recommend using a vimdiff friendly color scheme like "greens" which is described here :
Here's how to set yourself up to use vimdiff as Git's difftool.