When reading a diff or a range-diff, it can be handy to have colorized output because it's easier to distinguish between the different lines. This is especially true of range-diff, which has at least 6 possible line prefixes. The git [diff|range-diff] commands can generate a colorized diff/range-diff and display it in the pager, but they are unable to colorize an existing diff/range-diff in text form (say, a patch or range-diff from an email).

Many editors support colorization of diffs, but there is little support for range-diff. Adding an option to accept a diff file and colorize it would be a big productivity win for those who'd like a CLI-friendly, editor-agnostic way of view diff files.