width=1000 truecolor-test

#!/bin/bash
# Based on: https://gist.github.com/XVilka/8346728

awk -v term_cols="${width:-$(tput cols || echo 80)}" 'BEGIN{
    s="/\";
    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 "33[48;2;%d;%d;%dm", r,g,b;
        printf "33[38;2;%d;%d;%dm", 255-r,255-g,255-b;
        printf "%s33[0m", substr(s,colnum%2+1,1);
    }
    printf "n";
}'