4-color process printing is the method used to print full-color images such as photographs.
The concept behind the process is that the primary colors we all learned about in the first grade can theoretically combine to make up virtually any color. Recall that the primary colors were blue, red and yellow. The 4 process colors are the same with the addition of black – Cyan, Magenta, Yellow, and Black. Together they are known as CMYK.
When applied consecutively on a printing press and in the proper color separations and ink densities, they make up all the colors in an image. Take a look at the image below and then take a look at the color separations. Each color is printed one at a time and on top of each other. The order they print is black first followed by cyan, magenta and finally yellow.
If you take a look at a printed image of a photograph under magnification you will see the dots of each color making up the image. Some phones have strong enough zooms in camera mode to let you see them.