Fix colour coalesce

layers/transportation/network_type.sql

@@ -46,7 +46,7 @@ SELECT CASE
            WHEN network = '' THEN hstore('')
            ELSE hstore(
                ARRAY['network', 'ref', 'name', 'colour'],
-               ARRAY[network, NULLIF(ref, ''), NULLIF(name, ''), NULLIF(COALESCE(colour, ref_colour), '')]
+               ARRAY[network, NULLIF(ref, ''), NULLIF(name, ''), COALESCE(NULLIF(colour, ''), NULLIF(ref_colour, ''))]
            )
        END;
 $$ LANGUAGE sql IMMUTABLE