awk 'BEGIN{
    s="/\\/\\/\\/\\/\\"; s=s s s s s s s s;
    for (colnum = 0; colnum<77; colnum++) {
        r = 255-(colnum*255/76);
        g = (colnum*510/76);
        b = (colnum*255/76);
        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(s,colnum+1,1);
    }
    printf "\n";
}'

==========================
#!/bin/bash
# Based on: https://gist.github.com/XVilka/8346728 and https://unix.stackexchange.com/a/404415/395213

awk -v term_cols="${width:-$(tput cols || echo 80)}" -v term_lines="${height:-1}" 'BEGIN{
    s="/\\";
    total_cols=term_cols*term_lines;
    for (colnum = 0; colnum<total_cols; colnum++) {
        r = 255-(colnum*255/total_cols);
        g = (colnum*510/total_cols);
        b = (colnum*255/total_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(s,colnum%2+1,1);
        if (colnum%term_cols==term_cols) printf "\n";
    }
    printf "\n";
}'