Another css hexi hack (from jlilly)

(add-to-list 'auto-mode-alist '("\\.css$" . css-mode))
(autoload 'css-mode "css-mode" "Mode for editing Cascading Style Sheets")
 
;; make css colors their actual colors
(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 ()
  (interactive)
  (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)))
           (put-text-property
            (match-beginning 0) (match-end 0)
            'face `((:foreground ,(if (> 128.0 (hexcolour-luminance colour))
                                      "white" "black"))
                    (:background ,colour)))))))))