Blue is the hardest color to see as more light energy is required for a full response from blue-violet cones, compared to green or red. At a certain light level, a blue-violet color appears darker than green or red, notes the UCLA Department of Atmospheric and Oceanic Sciences.
The darkness seen in blue-violet is relative and applies to various color mixes, and the brightness of yellow stimulates two of the most reactive cone sets in the eye. The eye finds it difficult to focus on blue, green and red at the same time and focusing on blue-violet is particularly hard. This results in a haze caused by the blue-violet spectrum, which often leads to headaches. The blue-violet light can be filtered out by wearing sunglasses with yellow lenses.