use strict;
use warnings;

MAIN_LOOP:
for my $number_to_check (1 .. 20) {
    for my $colour ('red', 'green', 'black') {
        next MAIN_LOOP if $number_to_check % 3 and $colour eq 'green';
        print "$colour-$number_to_check ";
    }

    print "\n";
}