Visible light is the part of the electromagnetic spectrum that our eyes can detect. It turns out that this is a tiny sliver of the entire electromagnetic spectrum. We typically describe the photons in this range by their wavelengths, which span from approximately 380 to 700 nm. It is important to recognize that the cutoff is based on our biology, and not some larger physical principle. There are other creates in the animal world that can detect wavelengths that go outside of what we can see, and so their "visible light" would be a different range.

The order of the colors from long to short wavelengths is typically described using the mnemonic ROYGBIV. This stands for the following colors (and their approximate range of wavelengths):

You might notice that the I is missing. The letter I stands for indigo, and most people don’t distinguish it as a color that’s separate from blue or violet. It mostly remains for historical reasons, as this was how the color spectrum was first organized, and sometimes it’s difficult to change something once it has been established.

None of these colors are "real" in the sense that there are no well-defined boundaries on these colors. In fact, what we perceive as color is a complex biological and psychological system for using our eyes and brains to differentiate between different wavelengths. While physics certain plays a role in bringing us color, our brains play a gigantic role in how we actually perceive it.

Basically, your eyes have two different photon detectors (photoreceptors), one for low light called rods, and one for bright light called cones. It turns out that cones are completely responsible for seeing colors, which explains why low light situations are typically experienced with a muted color range. (The light is low so only the rods get activated.) Among the cones, there are three different types of detectors, one for each of the colors red, green, and blue.

But how can we see so many colors if we only have receptors for three colors? Each color receptor is activated by a range of wavelengths, and not a single one. The colors are just the ones where each type reaches its peak level of activation. When we perceive other colors, it’s because our brains are trying to interpret a mixed collection of signals. As the difference cones are activated to different levels, our brain comes up with a way of representing that activiation visually for us, and those are the colors that we see. One of the challenges of dealing with human color perception is that there is no guarantee that the way we perceive a color is the same way that someone else would perceive them. The easiest example of this to understand is the case of color blindness, where a person’s brain is unable to distinguish between photons of different wavelengths that other people’s brains are typically able to do.

So even though we will walk about colors as if they are concrete and specific objects, it’s important to keep in mind that it’s really much more complicated than that.