Great question! Overall, this is a quite a complicated question, without a straightforward answer, as the issue deals with a mixture physics, biology, psychology, and philosophy—but I will do my best to answer.

Light—or more correctly, the electromagnetic spectrum—is a continuous range of all the possible frequencies of electromagnetic radiation. Visible light (or, the light that humans can see) covers just a tiny, tiny range of the electromagnetic spectrum, from wavelengths of about 400 nanometers to 750 nanometers. Since we can only see light in this range of frequencies, this means that humans cannot perceive the vast majority of light in the universe. Now, when electromagnetic radiation interacts with an object, the light can be reflected, scattered, absorbed, or transmitted through the object.

The light that leaves the object will depend on the spectrum of light that is illuminating the object, the reflective properties of the object, the angle at which you view the object, and the other nearby objects. When you look at, for example, a pumpkin that is illuminated by white light, you perceive that the pumpkin is orange, because orange light (~620 namometer wavelength) is emanating from the pumpkin to your eyes. However, your eyes can also perceive a mixture of light to be the same color as that pumpkin.

For example, television screens only produce red, blue, and green light. So how do you see the color orange on a TV? The TV screen emits the correct mixture of red and green light, and your brain perceives that mixture as the same color of the pumpkin. So you see, our brains may perceive many different combinations of the electromagnetic spectrum as the same color.

In general, color is a perceptual property of how our brains interpret a small portion of the electromagnetic spectrum. Over many years of human evolution, different people and cultures have assigned color to be a designation of how an illuminated object stimulates the rod- and cone-cells in our eyes, and how our brains interpret this stimulation. Spectra that stimulate our light receptor cells in a similar way are grouped together and designated as a color. Each specific color that humans can see may result from many different combinations of the electromagnetic spectrum. So, “color” is the best way for humans to group together things that stimulate our bodies in a similar fashion, as we cannot perceive the full detail of how light interacts with all objects in our world.