The rather forthright Captain Disillusion (previously) explains how the human eye processes light wavelengths from the electromagnetic spectrum and converts them into what we know as color. The Captain further explains that the part of the spectrum we view is very, very small and that color does not exist outside the brain.
The most important thing to recognize about color is that it doesn’t actually exist. It’s true electromagnetic energy exists all around you. And if we sort it by wavelength, your vision is sensitive to a small sliver of that spectrum. But the sliver isn’t literally filled with the vibrant hues shown in diagrams, it’s just the waves. Colors don’t exist outside your brain.