Which are the bright terminal colours?

termcolors.sh

#!/bin/bash

get_color_from_palette() {
    # Query the terminal for the current value of a palette entry.
    # Prints three decimal numbers from 0-65535 representing the red, green and
    # blue components of the given palette entry.
    local paletteNum="$1"

    # This function only works in xterm and (some) derivatives
    if [ "${TERM:0:5}" != "xterm" ]; then
    return
    fi

    # Issue the query
    echo -n $'\e]4;'"$paletteNum"$';?\e\\' > /dev/tty 2> /dev/null ||
    return # Assume we have no tty to talk to, give up.

    # xterm responds with a string terminated with ^G, if it responds at all.
    read -d $'\a' -s -t 1 < /dev/tty

    if [ "$REPLY" == $'\n' ]; then
    # We got no response, give up.
    return
    fi

    # Strip the escape-code bits off the beginning; read already changed the
    # terminating ^G to a safe newline. We assume the response is of the form:
    #
    #    rgb:RRRR/GGGG/BBBB
    #
    # where RRRR, GGGG and BBBB are hex-digits.
    local raw_result=${REPLY#*:} # strip off everything up to the first :
    local result=${raw_result//\// } # change "/"s to " "s

    local component
    for component in $result; do
    echo $(( 0x$component ))
    done
}

get_palette() {
    # Query the terminal for the current palette.
    # Returns one line per palette entry, each containing three numbers each
    # from 0-65535 representing the red, green and blue components of the given
    # entry.
    local index
    local pal_size=$(tput colors)
    local raw_result
    local result
    local component

    # Check we can actually talk to the terminal, first.
    echo -n "" > /dev/tty 2> /dev/null || return

    # Run this in a subshell so we can do the redirections once and hopefully
    # not miss any bytes.
    (
    # Send our query
    echo -n $'\e]4;'
    for (( index=0 ; index < $pal_size ; index++ )); do
        echo "$index;?;"
    done
    echo -n $'\a'

    # Read our responses
    for (( index=0 ; index < $pal_size ; index++ )); do
        # xterm responds with a string terminated with ^G, if it responds
        # at all.
        read -d $'\a' -s -t 1

        if [ $? -gt 0 ]; then
        # We got no response, give up.
        echo "no response" >&2
        return
        else
        raw_result=${REPLY#*:} # strip off everything up to the first :
        result=${raw_result//\// } # change "/"s to " "s

        for component in $result; do
            echo -n "$(( 0x$component )) "
        done
        echo
        fi
    done
    ) #< /dev/tty > /dev/tty
}

get_luminance_from_color() {
    # Return the luminance of a color as a number 0-65535.
    local red=$1
    local green=$2
    local blue=$3

    # Colour weights from Wikipedia. Note bash doesn't have floats.
    echo $(( (2126 * $red + 7152 * $green + 722 * $blue) / 10000 ))
}

get_luminance_from_palette() {
    local color=$(get_color_from_palette "$@")
    if [ -z "$color" ]; then
    # can't get colors from this terminal.
    return
    fi

    get_luminance_from_color $color
}

get_bright_colors() {
    # Print a list of palette indices visible against a bright background.
    local index
    local colors=$(tput colors)
    for (( index=0 ; index < colors ; index++ )); do
    local luma=$(get_luminance_from_palette $index)
    if [ -z "$luma" ]; then
        # can't get colours from this terminal.
        return
    fi
    if [ "$luma" -ge 16384 ]; then
        tput setaf $index; tput setab 0 
        printf "%s" $index
        tput sgr0
        printf " "
    fi
    done
    echo
}

get_bright_colors