He had cataracts:
After years of failed treatments, he agreed at age 82 to have the lens of his left eye completely removed. Light could now stream through the opening unimpeded. Monet could now see familiar colors again. And he could also see colors he had never seen before. Monet began to see–and to paint–in ultraviolet.
The paintings above are of the same scene. The red and yellow version is painted as seen through his left eye, limited to the wavelengths allowed by his cataract. The painting on the right is deep blue and violet, as seen through an eye with no lens. Who can imagine how those colors appeared to his eye while being mixed on his palette?