In transparent watercolor, artists work with thin, literally transparent, washes. Nevertheless, you can quite easily paint cadmium red, orange, or yellow spots over other watercolors by using very little water and more paint than usual, because cadmium colors are sufficiently opaque. An experienced aquarellist saves himself a great deal of trouble by planning his work according to such possibilities. In all painting media, the use of transparent color as glazes adds depth to your work. The beauty of paintings by old masters is due to a very large extent to the fact that those artists executed an underpainting first, often in tempera, then glazed it in oils.
Many artists now do a casein underpainting, and glaze it in oils or in polymer.
The degree of opacity also has a bearing on the mixing of colors. Certain opaque hues are amazingly powerful. For example, you would need an incredible heap of white to make cadmium yellow noticeably lighter. On the other hand, the slightest touch of white is enough to turn alizarin crimson into a pale pink, in any medium. Some manufacturers indicate the opacity, transparency, or semi-opacity of each color on the label in oil paints.