The monkeys approached objects they had been trained to avoid. Eyes don't see. That's hard to say. The perception of color demonstrates how the brain turns sensory information into perception, how the mind's eye makes sense of the world. We interpret, rather than see, color.
Maple leaves are actually red and green in all seasons.
When light bounces off the green pigments in the chlorophyll, it blinds us to the red. The chlorophyll leaches out of the leaves when the weather turns cold in the fall, and we finally see the red. What humans see as red, another animal might see in shades of black and white or as a completely different color. Some animals see colors humans can't. Culture makes a difference in color perception, too.