Answer:
color of light
Step-by-step explanation:
The independent variable is the variable that is supplied and often varied/manipulated by the researcher during the course of experiments. It is varied/manipulated in order to see the effect it will produce on another experimental variable - the dependent variable.
In this case, out of all the variables supplied by Mary, only the color of light supplied was varied - red, white, green, and blue; other variables supplied were kept constant. Hence, the independent variable is the color of light.