Basta adicionar essas linhas aos seus arquivos .emacs (ou .emacs.d / init.el)
;; Customizing colors used in diff mode
(defun custom-diff-colors ()
"update the colors for diff faces"
(set-face-attribute
'diff-added nil :foreground "green")
(set-face-attribute
'diff-removed nil :foreground "red")
(set-face-attribute
'diff-changed nil :foreground "purple"))
(eval-after-load "diff-mode" '(custom-diff-colors))
(minha configuração completa do emacs pode ser encontrada em github: https://github.com/clarete/dotfiles/tree/master/.emacs.d )