test-truecolor.sh

#!/usr/bin/bash
awk -v term_cols="${width:-$(tput cols || echo 80)}" 'BEGIN{
    for (colnum = 0; colnum<term_cols; colnum++) {
        r = 255-(colnum*255/term_cols);
        g = (colnum*510/term_cols);
        b = (colnum*255/term_cols);
        if (g>255) g = 510-g;
        printf "\033[48;2;%d;%d;%dm", r,g,b;
        printf "\033[38;2;%d;%d;%dm", 255-r,255-g,255-b;
        printf "%s\033[0m", substr("||",colnum%2+1,1);
    }
    printf "\n";
}'