It depends on the sense in which you are using the word colour.
In physics, the real phenomena which corresponds most closely to "colour" is electromagnetic frequency
However, the eye itself does not simply detect the specific electromagnetic frequency of what it can see. Rather, it has a set of so-called cones which respond to a range of frequencies. The response of each cone is strongest at an ideal frequency, and attenuates the further the frequency is off from the ideal.
In most humans, there are three types of these cones, which have ideal frequencies that roughly correspond to red, green, and blue. In the "colour blind", one or more of these cone types (typically the red or the green type) are missing. Exceptionally in humans, some have four types of cone which extends the ability to perceive colours, and animals can have a significantly different configuration to humans.
Any given colour tends to activate all the cones to varying degrees according to the frequency. A red light will activate the red cone to a high degree, the green less so, and the blue less so again. A yellow light will tend to activate red and green to similar degrees, but the blue less so. The brain compares the relative strength of signals from all three to determine the colour.
The effect of missing cones is that the ability to precisely differentiate between different frequencies is lost. The person can still see broadly as normal, but some examples of colours that strike the normal person as obviously and vibrantly different (like red and green) begin to look similar - one colour will merely seem like a duller version of another colour, since a person will have fewer types of cone from which to draw the distinction.
Another issue is the status of the colour "white". Humans often tend to perceive or conceive this as a specific colour, but at the physical level it is a colour chord consisting of multiple or all colour frequencies across the visible spectrum.
This colour chord system is also the reason why colourblind people struggle to distinguish, since more than one physical frequency profile is capable of producing the same perceptual response - in practice, normal humans can be made to perceive white light simply by showing them pure mixtures of red, green, and blue, and they can be made to perceive yellow by showing them either pure yellow or by showing them a chord of red and green.
Black, too, is special in that it corresponds to the absence of detectable light, not to the frequency of any light.
Also, the naming and categorisation scheme for colours is conventional or specific to normal human biology, and doesn't closely correspond to any objective physical schema.