You might've also changed your color theme and forgotten. You can pipe through less (or something else that doesn't understand the control code) and look for ^[[31m is red; ^[[32m is green; ^[[34m would be blue (although ^[[36m is kindof a light blue/cyan) to tell if you've reconfigured your terminal, or you've reconfigured git's default colors, but if you haven't done either you might want to get your color-vision checked!
[color "diff"]
plain = normal
meta = bold
frag = cyan
old = red
new = blue
I guess I changed to match the red-blue color convention I'm used to from latexdiff.