The choice of color here is not obvious to me. For example, greener could mean more money, i.e. more expensive. Also red connotes debt/negative in accounting, while black is surplus/positive. If you're eating a tomato, red is good and green is bad (unless fried I guess).

>What percent of the rest of the population is confused by not using red & green?

Presumably none if you build your site in an accessible manner. It costs basically nothing to accommodate the 1/12 of the population that has color-blindness.