How to get background color in Emacs?

If you know how to find out from the terminal, you can use that selfsame command to find out from Emacs.

In my case, I'd make a script like this:

#!/bin/zsh

cat .Xresources | grep 'URxvt\*background\:' | cut -d" " -f2

(Note: -d is to set the field delimiter, -f is to set what field is to be shown: the first field is 1, not 0)

The command looks the way it does because .Xresources, the file that sets the background color, looks like this:

# ...
URxvt*background: black
# ...

Make the script executable (chmod +x), and put it in your PATH (echo $PATH).

If the script is called what_bg, in Emacs, M-x shell-command RET what_bg.

Edit (in response to comment):

See if this works. I tested it from Emacs, and in urxvt, xterm, and rxvt. While it is more portable than the first script, it assumes .Xresources configuration (which is, while not uncommon, obviously not everywhere).

I'm starting to wonder, though, why you need this to begin with?

And, if you indeed need it, can't you just look on the window to determine its color?

Anyway, the script:

#!/bin/zsh

terminal_emulator_parents=`pstree -As $$`
tep_list=`echo $terminal_emulator_parents | tr -s "-" | tr "-" " " \
          | tac -s' ' | tr '\n' ' '`

found="false"
for process in `echo $tep_list`; do
    if [[ $process =~ ("urxvt"|"xterm"|"rxvt") ]]; then # here: add all
        found="true"                                    # terminal emulators
        break                                           # configurable
    fi                                                  # (and *configured*)
done                                                    # in ~/.Xresources

if [[ $found == "true" ]]; then
    echo -n "$process: "
    cat ~/.Xresources | grep -ie ^$process'\*background\:' \
                      | tr -s " " | cut -d" " -f2
else
    echo "Couldn't determine the terminal emulator."
fi