Every color experience you have had involves light — colored light, that is, varied wavelengths of electromagnetic energy. When colored lights are influencing what we see, an additive color mixing paradigm has to be applied.
Most of your daily color experiences also involve pigments — materials that interact with light, selectively absorbing some colors and reflecting others. When objects in nature or man-colors objects using pigments, paints, inks and dyes are involved, then we have to think in terms of the subtractive color paradigm.
As it turns out, you often have to pay attention to both additive and subtractive color issues. When you design color, you've got to pay attention to where the color is coming from — in what ways are pigments and surfaces reflecting colors, and in what ways are colored light sources influencing what you see?