An alternative would be to let the r.g.b/24 subnet produce the color. That is simple, direct and irreversable, but still meaningful to the eye.

sub ip2rgb {
    my @ip = @_;
    push @ip, (0) x 3;
    @ip[0..2];
}
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That takes the ip quad as a list of octets. The Socket::inet_*() functions can be used to convert address forms.