Question: Why are there so many paint colors?

One reason there are so many paint colors is that vendors use the same pigments in more than one color. In other words, while there are several pure colors made from unique pigments there are many more colors that are premixed by vendors. For example, Winsor & Newton Phthalo Blue is what I call a "pure" color made only from one pigment (copper pthalocyanine). On the other hand, Winsor & Newton Indigo is a premixed color that includes both Phthalo Blue (copper pthalocyanine) and Ivory Black (carbon black).

Premixed colors are created either for convenience of the painter or to simulate more expensive pigments with a less expensive pigments. Here again Winsor & Newton Cadmium Red Hue is a mix of two pigments, naphthol AS and naphthol carbamide. There is no cadmium (that is, cadmium sulfides or sulfoselenides) in this color although the name makes clear reference to this pigment.