#!/usr/bin/perl

my $red    = undef;
my $yellow = undef;
my $blue   = undef;

sub do_something {
    # some computation
    $_[0] = (some new value);
    $_[1] = (some new value);
    $_[2] = (some new value);
}

... 

do_something(color1, color2, color3);


##</code><code>##


sub do_something {
    my ($red, $yellow, $blue) = @_;

    # some computation
    $red = (some new value);
    $yellow = (some new value);
    $blue= (some new value);
}