No two people ever see the same colour. Some may say this is because of psychological factors, others, well.. who knows. This is my reason as to why two people never see the same colour.

There are three different cones in our eyes. They are characterized by the photopigment that they have. In short, they are red, green and blue. Now, this is NOT to say that every blue cone is the same. There are differences, such as the number of photopigments actually in the cone. Now, why would this cause any problems you ask? Well, when blue light is shined into our eyes, these pigments become activated and send signals through complicated processes that I will node later, to the brain. Now blue cones that have different pigment counts will be activated differently by the same light. Hence, the signal that is sent to the brain will be different.

Furthermore, the distribution of these cones and rods are not random, but are not fixed either. There is a pattern, there are more cones near the fovea but more rods in the periphery. The number of cones and rods differs from individual to individual. So a light, incident on the same piece of the retina will activate different amounts of receptors.

So, a single light beam of PURE blue color will activate different amounts of blue cones in different people, and the signal to the brain will be different based on the cones activated and the amount of photopigment that are actually activated. Now that is just for one colour. When you factor in different shades, you are factoring in different ratios of cones activated and those two differ from individual to individual. Hence, no two people see the exact same colour.