At work we do code review by getting emails with the diff of every commit to our SVN repository. One day I wondered whether there was an addon for Thunderbird to get colored diffs - and sure enough, there was:
It features various display modes, e.g. simple coloring of unified diffs:
Or side-by-side diffs:
As you can see it will also display whitespace characters, if you want to.
Moreover, you can set the tab width to your preferred value.
You can find more screenshots in the wiki of colorediffs.
Yet again, another useful Thunderbird-addon making my life a little bit easier.