I want to say that each month also has a unique color. So the two color combo can easily be translated to the month/year your tag expires. A friend of mine drove for a long time using stickers he had peeled off of other cars. As long as he kept is driving legal, he would pass the "color checks". However, once he did something that gave a LEO cause to run his plate... clearly he was busted for it. Kind of interesting when you think that these cameras would have caught him straight away.

Another scenario, when people have paid for their registration but for one reason or another have forgotten to place the sticker on their plate. Those people would not get hit with these camera since their plate would not come back as expired. They would still fail the "color check" which usually can be explained to the LEO. If not, it is usually just a fix-it ticket.