Read a color name or RGB triplet.
Completion is available for color names, but not for RGB triplets.
RGB triplets have the form "#RRGGBB". Each of the R, G, and B
components can have one to four digits, but all three components
must have the same number of digits. Each digit is a hex value
between 0 and F; either upper case or lower case for A through F
In addition to standard color names and RGB hex values, the
following are available as color candidates. In each case, the
corresponding color is used.
* `foreground at point' - foreground under the cursor
* `background at point' - background under the cursor
Optional arg PROMPT is the prompt; if nil, use a default prompt.
Interactively, or with optional arg CONVERT-TO-RGB-P non-nil,
convert an input color name to an RGB hex string. Return the RGB
If optional arg ALLOW-EMPTY-NAME is non-nil, the user is allowed
to enter an empty color name (the empty string).
Interactively, or with optional arg MSG non-nil, print the resulting color name in the echo area.