sub write_graffiti {
    my $self = shift;
    my %params = @_;
 
    $self->{colour} = $params{colour} if defined $params{colour};

    # spray paint wall with the colour,
    # whether new or existing
}

##</code><code>##

sub colour {
    my $self = shift;
    my $colour = shift if @_;

    $self->{colour} = $colour if defined $colour;

    return $self->{colour};
}