Whoa, I never thought about that. The colorblindness thing is exactly why we haven't tried to remove the +s and -s, but if there are indeed colors that we could attempt to use, that'd be awesome.
The diff convention of marking inserted and deleted lines by + and - provides a "channel" of info showing text comparisons. If the + lines were also blue-green and the - lines also red-violet then there's two channels conveying the same information.
Using colors or not, we'd definitely keep the +/- markers.
The highlighting methods are distinct, but line information is the same. The +/- notation will still be useful on a monochrome monitor or printed page. On a spiffy new 2500x1600 display, I'd personally prefer the color output to quickly find what's changed.