He could actually see ultraviolet light. You can see this in his painting. He did an experiment painting two of the same picture, one using only his left eye, the other using his right. One is completely orange and yellow, the other is blue and violet.
The color differences are actually so severe. And he did another set of the same painting to show us what colors he saw throughout his life. He painted the same bridge I believe. It's amazing. The cataracts made him see more yellows and less blues and greens.