I try and keep a simple colour scheme definition for my web site using some CSS variables:

--theme-fg: #2b2d3a;
--theme-bg: #fffbef;
--theme-alt: #ffa500;

rainbow-mode of course helps to show the colours in situ, but sometimes if I am changing the colour scheme I use list-colors-display and then copy and paste the hex value. But should I use the name instead?, it is tempting as the colour would be very apparent and readable in the CSS file but then it is difficult to just incrementally tweak a hex value to get the colours to look just right. I also like to use orange, well I always seem to revert back to it but I can never remember the hex value.

There is an in-built function in emacs which can help me out with all this called css-cycle-color-format and is bound to C-c C-f

With the point over the colour, potentially three different CSS color formats are cycled, by name (if possible), hexadecimal, and rgb()/rgba().

For example I could cycle through orange as thus:

--theme-alt: orange;
--theme-alt: #ffa500;
--theme-alt: rgb(255, 165, 0);
--theme-alt: orange;

What about a colour / hex value in a non CSS file I hear you say, well although css-cycle-color-format is bound to css-mode-map it can be called for other files and in other modes. For example in init.el calling css-cycle-color-format seems to work so I guess it could just be rebound to any mode-map as desired.

Also if you have a CSS file with a profusion of hex colour values the css-cycle-color-format command could find the name value for you, for example as a test, putting in a colour that is obviously red :

--theme-alt: #ff0000;

cycles to:

--theme-alt: red;