use Carp qw(croak);

sub set_colour {
    my $this = shift

    croak 'set_colour requires an argument'
        unless @_;

    $this->{colour} = shift;
}

# if you like combined setter/getters
use Scalar::Util qw(reftype);

sub colour {
    my $this = shift;

    if ( @_ ) {
        my $arg = shift;
        croak 'Illegal value for method colour'
            unless reftype $arg eq 'ARRAY';
        $this->{colour} = $arg;
    }

    return @{ $this->{colour}||[] };
}