Red light

Red light means do not buy. It is presumably what you see when you swipe the barcode of a product whose price premium plus its retail price is not cheap enough to make you buy it.

It assumes that some price and price premium data have been compared, and that there is another option that is better that can be substituted.

A yellow light is a less strict and more likely way to signal a problem with purchasing a given product. A green light just means "buy".