If the clothing of the saint was the only light area in the funeral of st. bonaventure, the viewers eye would not be easily drawn to any other areas of the composition.

This sentence is true.
In the composition, we can see people carrying the body of St. Bonaventure to bury him because he died. We can see that he is dressed in white, and if he were the only person painted with light colors, the viewers wouldn't really focus on any other part of the painting. 
I've attached the painting so you can see it.