Many countries use red and green - but not all of those countries work like the UK in that customs have very little interest in what individuals bring in, so long as it's not illegal drugs etc, and airport customs posts are often completely unmanned. They're more interested in larger smuggling operations and use intelligence to target them.

I suppose it's not dissimilar to the "honour system" on most European city train and bus networks, in that you are trusted to ensure you are complying with the rules, but if you are caught on random inspection and you are not complying, the penalties are severe.

Neil