A substance is known to reflect red and blue light. What color would it have when it is illuminated by green light?

A substance known to reflect red and blue light, and illuminated by green light, will have all the three primary light colors. The color it would reflect is white.

Explanation:

In visible light spectrum, a combination of red light and blue light gives magenta. Also a combination of red and green lights gives yellow light. and when green and blue lights mix the resulting light color is cyan.

However, if these three primary colors mix in equal proportion, we will have a neural color or white color and in the absence of these primary colors we will have black.

Thus, a substance known to reflect red and blue light, and illuminated by green light, will have all the three primary light colors. The color it would reflect is white.