Short answer
The colours we see are determined by the light-sensitive cells within the eye, but also by our brain. One eye is not like the next. Furthermore, there are large discrepancies between the brains of each individual, not to mention that names for colours are a learned naming system, and certainly not taught in the same way for everyone. This can therefore create some large differences of opinion about the colour of a certain object; ‘what a pretty, blue vase’, ‘what are you talking about, that vase isn’t blue, it’s green!’. Sound familiar?
Longer answer
Is that vase blue or green?
First of all, the light that travels from the vase to our eyes is technically not coloured. Colours are generated, so to speak, through a neurological process in our brain. The light that is observed by our eyes does have many varying attributes, such as the wavelength of the light, which is an important aspect of the eventual perceived colour. Certain light-sensitive cells in our eyes, called cones, react very specifically to a particular wavelength of light. In actuality, this makes us very sensitive to colour. In principle, we could differentiate a million different colours from each other, when placed side by side. Yet we only have a handful of words for colours. That is the crux of the argument and the source of many debates. How we associate names to specific colours depends on many factors, such as the way this naming of colours has been taught to us. There is by no means a decided, defined line where one colour stops and another takes over. Especially for colours that are between two groups of colours, such as red and orange, or green and blue, you get many different opinions. The colour “turquoise”, for instance, is somewhere between green and blue, which can lead to strong differences of opinion (‘clearly blue’, ‘no, no, green!’). But think on this; when one person clearly states “blue” and another ‘green’, they can, in fact, still be seeing the same colour, they just have different names for it!
Environment and culture. It could also be that they both say it is ‘blue’ but still see slightly different colours! The number of names for colours is very dependent on the environment and the culture. In some cultures, there is no word for blue at all. According to a researcher, there is a people in Namibia that do not have a word for the colour blue, but have many shades of green. On the other hand, there are many different names for shades of light blue and dark blue in Russia. Researchers believe this makes them more sensitive to distinguishing between these shades of blue.
Physical causes. Differences of perception in colour can also be caused by a slightly different level of sensitivity in the receptors of the eye, or even by the complete absence of specific light-sensitive cells. One such example is colour-blindness. Red-green colour-blindness is the most common version of this (especially in men). There are also people that have an extra receptor in the eye, which allows them to pick up more nuances in colour. There are also some animals that can perceive more colours; some snakes, for instance, can see infrared, and birds and bees can see ultraviolet light.
Figure 1: The famous dress that some people perceive as blue-black, while others see as yellow-white (source: https://en.wikipedia.org/wiki/The_dress)
Expectations. A final factor that often goes unnoticed and plays a big role in colour perception, is the background lighting. The colour of an object shifts rather strongly with the change in colour of the lighting. Yet, that often goes unnoticed. After all, our brain compensates for it; a phenomenon called colour constancy. That can sometimes lead to bizarre situations, especially if we don’t take into account what the background lighting looks like. This was proven a few years ago by famous picture of ‘the dress’ (see above, figure 1). Some people saw it as blue and black, while others saw it as white and yellow. This is due to the ambivalence of the colour of the background lighting. Subconsciously (!), our brain assigns a certain colour to the background lighting. For instance, if our brain were to assume that there was more blue lighting, it would compensate for that, and the dress would be perceived as more white and yellow/gold (the ‘blue in the dress’ therefore comes from the lighting and is not part of the dress’s colours). Other research has proven that our expectations do influence what colour an object is perceived: a banana depicted in tints of grey is still perceived as a bit yellow! We see what we expect to see. In this instance, the colour you see really is all in your head!
Further Reading?