Css-color.el propertizes colours in a CSS stylesheet found by font-locking code with a keymap. From that keymap, you can easily adjust values such as red green and blue, hue, saturation and value, or switch between different color (space) notations.
It supports all ‘css-colors’, so hex, rgb(), hsl() and even HTML color names (although I wouldn’t use them myself, it is nice to be able to quickly convert those), can be used and switched between.
The rgb() notation can be expressed either in percentages or in values between 0-255.
You can cycle between the different formats (with SPACE), so that it is possible to edit the color in hsl mode (which is more intuitive than hsv, although hsv has its merits too), and switch back to rgb or hex if so desired.
With point on a color, the keys - and = to are bound to the down and up functions for channels (or ‘fields’). Toggling percentage in rgb() is done with the % key (not sure if that is wise though). The TAB key is bound to go to the next channel, cycling when at the end.
Rainbow mode also provides this functionality, and is available on GNU ELPA.
Css-color.el as distributed with nXhtml.
You may want to use the latest version that comes with nXhtml instead. You can grab it directly from the repositories using the web interface. It has no dependencies of nXhtml. (It had before, but that was a mistake.)
One can use the the version of Css-color.el distributed with nXhtml only IF it can be found. Where is the link to css-color.el distributed with nXhtml without the entire nXhtml package tree, bzr checkout etc. Without provision of a link for just css-color.el users are still required to download all of nXhtml and then extract the relevant library. --mon key