White or visible light while it seems to be colorless, contains all the wavelengths of in the visible spectrum of electromagnetic energy. Visible light is electromagnetic radiation with a frequency lower and longer wavelength than ultraviolet and higher frequency and shorter wavelength than infrared. Visible light and near infrared light is typically emitted and absorbed by electrons moving from one orbital zone to another in atoms and molecules.
White light is the observation of 'all' electromagnetic energy with wavelengths between 400-790 terahertz. When visible light is passed through a prism it is refracted to show the visible spectrum of colors "ROYGBIV". Red, Orange, Yellow, Green, Blue, Indigo, Violet. When looking at a rainbow the violet light has the higher shorter wavelength while the red has a longer wave length. Thus the bracketing of visible light by ultraviolet and infrared. (Infrared cameras are able to pick up this lower frequency light and bump it up to our visible spectrum.)
Subtractive Light. In school we're generally tough the primary colors are Red, Yellow and Blue. This was the first discovery of primary colors. Primary colors are colors than can't be made using other colors of visible light, but with the primary colors all other colors of visible light can be made. Red, Yellow and Blue are the simplified saying for the subtractive primaries Cyan, Magenta, and Yellow. Subtractive primaries were the first primary colors discover long ago with paint and dyes. Subtractive light is light that is reflected. So when you look at your favorite green shirt or the red on your Ferrari. What's happening is all the visible light that hits those objects is absorbed and converted into heat. While the green from your shirt or red from your Ferrari is the visible light being reflected back out. Also explaining if white pigment reflects all light, and absorbed light is converted into heat, your black shirt absorbing all light converting it to heat, is going to be much hotter than your white one reflecting light. Essentially working with pigments is creating a scientific color trap, you learn to trap the colors you don't want when mixing Cyan, Magenta, and Yellow.
Where does this come into play for a photographer? When you're going to print your Printer is going to be working in CMYK. Cyan, Magenta, and Yellow are used to create every color and the K stands for black which is used for shading. As for the K for black and not B? I can only speculate they didn't want to confuse people assuming it was blue. Black has a K and none of the ROYGBIV colors do, and CMYK sounds cooler than CYMB.
So in short when printing you're working with subtractive colors and the primary colors are CMY + K (black) for shading.