The light in a rainbow is made up of many more wavelengths than red, green, and blue. Indeed, every color and shade in the rainbow is a distinct wavelength. Our eyeballs have three different kinds of color receptors. Each receptor responds to visible light, but more strongly to certain wavelengths. You can trigger any color that we can perceive using 3 wavelengths such as red, green, and blue (some other combinations can also be used). We can perceive many more wavelengths than that; we just can't distinguish them.