I hate those stupid tinted plastic license plate covers (c90 s6) and tinted headlight/turn signal covers (c90 s7). It is a good idea to stop for improper display of plates when there is only one red plate displayed, because often times people are running two cars on one plate. As MT1 stated, whatever feels like a good stop on any particular day. I stopped a guy who had all sorts of lumber sticking out of his pickup in excess of four feet without a red flag. He ended up having a warrant. I wrote him a warning for the c90 violation and locked him up on the warant.