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When I added the following lines to my .emacs file, the color highlighting in
magit diff is disappeared:
(defun font-lock-warn-todo ()
0 'font-lock-warning-face t))))
(add-hook 'font-lock-mode-hook ; Make fixme tags standout
Without font-lock-mode-hook colors in diffs works well.
You should add font-lock-warn-todo to the appropriate programming and text mode hooks, not to font-lock-mode-hook.
Just for you I have written a minor mode to do so hl-todo (just kidding I wrote it for myself). You can find it here: https://github.com/tarsius/hl-todo. The library header contains pointers to other existing implementations and the relevant page on the Emacswiki.
Edit: it seems I can never get these damn markdown links right - total org-mode damage.