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