How does the tmux color palette work?

You can get a list with this bash snippet:

for i in {0..255}; do
    printf "\x1b[38;5;${i}mcolour${i}\x1b[0m\n"
done

Then use colourxxx with tmux.

I found this image to be enlightening.

In Subversion (what will be tmux 1.5) you can also use #abcdef hex-style colours which are mapped to the nearest 256 colour palette entry. You need quotes as it's treated as a string, whereas regular color names are treated as named constants. Also note that 3-letter shorthand (#f00) is invalid.

Example:

set pane-active-border-bg red # no quotes for name
set pane-active-border-bg "#ff0000" # quotes for rgb

Before tmux 3.2 (released in April 2021), tmux only supported the international (British) spelling for the 256 colour palette, e.g.

"colour121"

as opposed to the American spelling that drops the u

"color121"

If you're using tmux 3.2 or later, you can spell it either way.

Building up on @cYrus' answer, I wrote a script to break the output of the colors into N number of columns, where N is the first argument

# colors.sh

#!/bin/bash
if [ -z $1 ]; then
    BREAK=4
else
    BREAK=$1
fi
for i in {0..255} ; do
    printf "\x1b[38;5;${i}mcolour${i} \t"
    if [ $(( i % $BREAK )) -eq $(($BREAK-1)) ] ; then
        printf "\n"
    fi
done

Try it by saving it into a file called colors.sh, then ./colors.sh 4

Don't forget to chmod +x colors.sh first.