From https://stackoverflow.com/a/60108175/117471

tput_256colors

#!/bin/bash

tput init

end=$(( $(tput colors)-1 ))
w=8
for c in $(seq 0 $end); do
    tput setaf $c
    [[ $c -ge $(( w*2 )) ]] && offset=2 || offset=0
    [[ $(((c+offset) % (w-offset))) -eq $(((w-offset)-1)) ]] && s=$'\n'|| s='   '
    printf "tput setaf %3s%s" "$c" "$s"
done

tput init