White light is made up of all colors of light. If an object reflects all colors of light, then it looks white. When an object absorbs one color of light, say red, then all of the colors of light are reflected except the red light that was absorbed. Because of the way we perceive color, we see the reflected light as the complementary color of the absorbed color. In this case, green.

The same effect happens when an object absorbs green light. All of the light except green is reflected back and the object appears to be red in color.

If you are interested in the way we perceive color, I recommend reading (or asking Scienceline) about color theory and subtractive color. The set of commonly-taught primary colors, red-yellow-blue (RYB) is actually not the best set of primary colors. The set of cyan-magenta-yellow (CMY) (or for additive color, red-green-blue (RGB)), which you may have seen used for computer printers and displays, is actually better for scientifically describing color and explaining how we perceive it. CMY works much like RYB, but the primary and secondary colors are different. The complementary colors from CMY are also slightly different, but more accurately describe what we actually see.