WKA rule for numbers on RR kart.

There is no specific color in the rule. It just says " . . . must be in contrasting colors and easily readable by the scoring and race officiating personnel."
As long as you can read is probably what matters most. It was THE thing to do with numbers on a white background when we actually had the thankless jobs as "scorers". Transponders kinda gave us the numbers/background options nowadays.