Bash terminal 256 colors escape sequences. The following script display the 256 colors available on some terminals and terminals emulators like XTerm and GNOME Terminal.

256-colors.sh

#!/bin/bash
 
# This program is free software. It comes without any warranty, to
# the extent permitted by applicable law. You can redistribute it
# and/or modify it under the terms of the Do What The Fuck You Want
# To Public License, Version 2, as published by Sam Hocevar. See
# http://sam.zoy.org/wtfpl/COPYING for more details.

for mode in {1..7} 9 21; do
echo Mode: $mode
for fgbg in 38 48 ; do #Foreground/Background
	for color in {0..256} ; do #Colors
		#Display the color
		printf "\e[%s;%s;5;%sm %3s\t\e[0m" "${mode}" "${fgbg}" "${color}" "${color}"
		#Display 10 colors per lines
		if [ $((($color + 1) % 10)) == 0 ] ; then
			echo #New line
		fi
	done
	echo #New line
done
 
#exit 0

Above code is somewhat limited to combination of 256 foreground colors to 1 background color and 256 background colors to 1 foreground color.

Replacing tput setab <bg_color_num> and tput setaf <fg_color_num> should offer 256x256 color combinations.