Opaque and Transparent Colors.—Some colors have a jelly-like consistency when mixed with oil, others, the earths among them, are dense and opaque. We speak of them respectively as “transparent” and “solid” colors. These qualities, which divide the paints into two classes, have no relation to their permanency. As far as that is concerned you use them in the same way, as some transparent colors are safe and some fugitive; and the same with the opaque colors. The only difference is in the fact that, as a rule, the solid colors are better dryers. But you will notice that while you may mix these colors together as though this difference between them did not exist, in certain processes you use them differently….