Thank you Language Hound!
The sentence comes from an American book providing teaching techniques. This one belongs to the chapter about consequences, punishment etc., and the whole sentence is: We’ve all seen an elementary school student flop down and weep over a color change. But in Emilie’s system, four checks result in a color change (green to yellow or yellow to red). This enables her to address small behaviors consistently and to distribute checks without hesitation, without having to immediately resort to a card color change (which can be a big deal to young kids).
I thought about a semaphore system, or something like the cards of the referees in soccer. I just wanted to understand how it works exactly.