It is possible to teach Emacs to highlight HTML-style colour specifications a là #034fa3 in the colour they specify.

If you are using GNU Emacs from a recent (as of summer 2003) CVS (the one that will probably become Emacs 22.1) this is as simple as putting …

 (defvar hexcolour-keywords
      (0 (put-text-property (match-beginning 0)
                            (match-end 0)
			    'face (list :background 
				        (match-string-no-properties 0)))))))
 (defun hexcolour-add-to-font-lock ()
   (font-lock-add-keywords nil hexcolour-keywords))
 (add-hook <your favourite major mode hook> 'hexcolour-add-to-font-lock)

… into your .emacs.

You can use “#[[:xdigit:]]\\{6\\}” instead of “#[abcdef[:digit:]]\\{6\\}”, which will match ABCDEF digits too. RaffaeleRicciardi

Due to some quirks in the implementation of font-lock, this won’t work with Emacs 21.2 or 21.3. But you can use the package “hexcolour.el” (Lisp:hexcolour.el) to achieve this nevertheless. Read the commentary section for details.

I managed to get this working on Emacs 21.3, with some enhancements: contrasting colour for the text and support for 3 and 6 character hex codes. Very nice in css-mode.

  (defvar hexcolour-keywords
           (0 (let ((colour (match-string-no-properties 0)))
                (if (or (= (length colour) 4)
                        (= (length colour) 7))
                     (match-beginning 0)
                     (match-end 0)
                     'face (list :background (match-string-no-properties 0)
                                 :foreground (if (>= (apply '+ (x-color-values 
                                                                (match-string-no-properties 0)))
                                                     (* (apply '+ (x-color-values "white")) .6))
                                                 "black" ;; light bg, dark text
                                               "white" ;; dark bg, light text
  (defun hexcolour-add-to-font-lock ()
    (font-lock-add-keywords nil hexcolour-keywords t))


See also libraries Lisp:doremi-frm.el and Lisp:hexrgb.el, which let you manipulate hex colors and the Faces and Frame parameters that use them. – DrewAdams

Two more improvements, using luminance instead of average values, and fontifying color names (e.g. “blue” as well).

  (require 'cl)
  (defun hexcolour-luminance (color)
    "Calculate the luminance of a color string (e.g. \"#ffaa00\", \"blue\").
  This is 0.3 red + 0.59 green + 0.11 blue and always between 0 and 255."
    (let* ((values (x-color-values color))
           (r (car values))
           (g (cadr values))
           (b (caddr values)))
      (floor (+ (* .3 r) (* .59 g) (* .11 b)) 256)))
  (defun hexcolour-add-to-font-lock ()
    (font-lock-add-keywords nil
     `((,(concat "#[0-9a-fA-F]\\{3\\}[0-9a-fA-F]\\{3\\}?\\|"
                 (regexp-opt (x-defined-colors) 'words))
        (0 (let ((colour (match-string-no-properties 0)))
              (match-beginning 0) (match-end 0)
              'face `((:foreground ,(if (> 128.0 (hexcolour-luminance colour))
                                         "white" "black"))
                      (:background ,colour)))))))))


I like this. Just one change to the regexp for colour names:

(regexp-opt (x-defined-colors) 'words))

This keeps it from colouring parts of words like ‘redistribute’. Oh and for some reason I need to use the ‘append flag to font-lonk-add-keywords to get it working with Emacs 21.3. I really need to upgrade.


Good idea, added ‘words for easier copy&paste. – nschum