Human color vision is based on photoreceptors known as cones which are located in the back of the eye. These receptors process information about the light’s wavelength into electrochemical signals. Most people have three different photoreceptors (S-, M- and L-cones), thereby being sensitive for short, medium and long wavelength light. However, around 10% of the population are affected by color blindness with red-green color blindness being the most common type. People exhibiting the latter lack the respective photoreceptors for medium or long wavelength light and are not able to discriminate between green and red. On the other hand, there are also individuals specified as tetrachromats, who possess a fourth photoreceptor and are therefore more sensitive for certain hues within the normal color spectrum. So when regarding these two extremes – meaning that the physical part of color vision is altered in comparison to the normal case – one can actually say that colors can be perceived differently.
But what about people with normal trichromatic vision? As they share the same system for color perception which is wired in the same way within the brain, do they still vary in their inner experience of color? In the way their brain creates the image of a certain color? Or asked differently: is my yellow the same as your yellow? These questions are part of a broader philosophical concept termed qualia which refers to the subjective experience of a mental state. Because of this subjectivity in our inner perceptions and the limitations of our language to describe the latter (explanatory gap) – ultimately, because of the isolation of our minds, we will never be able to tell exactly if one person perceives a color in the same way as another person does.
Yet, there are some indirect hints that individuals perceive colors differently. For example, matching colors for clothing seems to be easier for some people than for others. Also, persons have different favorite colors and may attribute different moods with a specific color. A study on red-green color-deficient primates by Neitz and coworkers published in Nature in 2009 suggests that the way colors are created by the brain is not predetermined but can adjust to new conditions. Like red-green color-blind people male squirrel monkeys are dichromatic by birth because they lack long wavelength-sensitive photopigments. In a gene therapeutic approach, Neitz and coworkers introduced the human L-opsin gene to the monkeys by using modified viruses. In this way some of the M-cones could be converted into L-cones thereby establishing the necessary molecular machinery for detecting red and green light. Although the monkeys’ brains lacked the neuronal circuitry for L-cones they were able to exploit the new signals and to discriminate red and green against a grey background. In other words, the monkeys were able to see new colors. These results imply that there are no predetermined inner images of color ascribed to a certain wavelength. From this, one can interpret that the brain does not create colors in a default mode but that everyone develops his own unique way of perceiving colors. In the end, although we cannot be absolutely sure, we indeed might see colors differently.
Mancuso, K. et al (2009). Gene therapy for red-green colour blindness in adult primates. Nature, 461(7265), 784–7. doi:10.1038/nature08401