Read Color, Ruth Heller. There's a clear and colorful explanation of printing using four colors: Yellow, cyan, magenta and black. Or find another simple explanation to read (like this one, The Science of Light: Made from Dots).
Dotty Dots: You need a pencil, a piece of white paper, a yellow marker, a red marker, and a blue marker (preferably a cyan blue rather than a royal blue). Draw three circles with a gray pencil on the white piece of paper. In one circle, use the red and blue marker tips to make red dots and blue dots, as close together as you can get without overlapping. In another circle, do the same with the blue and the yellow markers, and use red and yellow dots to fill in the third circle. Look at the circles close up--can you see the separate colors? Now ask another person to hold up your paper as you look at it from across the room. Can you still see separate dots of different colors, or do they blend to look like one color?
Colors under the microscope: Look at a variety of pictures--photographs, postcards, books--with a magnifying glass and under the microscope. Do you see tiny dots used to make the rainbow of colors? Now look at a part of a printed photo or postcard that looks white. What do you see under the microscope?
Bonus Lesson: Read about Georges Seurat. His art was influenced by the science of color. Look closely at his pictures, and you'll see that they're composed of tiny dots of pure color.