The best kinds have no tendency towards either red or green. It is unaffected chemically by other pigments.
PRUSSIAN BLUE (iron cyanogen) is the second important blue and is essential to even the most limited palette. It is also known by its older names of Paris or Berlin blue. It is the strongest in tinting power of all pigments, the smallest traces make all yellow and even brown pigments a bright green. With red, however, it gives less clear mixtures, and with cadmium red a rich, deep black, so that the painter can dispense with any black pigment on his palette.
Unlike ultramarine blue (finest), which has strong covering power though it can also be used as a glaze, Prussian blue is a pure glaze color. In thick layers it assumes an unpleasant coppery tone and creates the effect of a hole in the picture plane. Mixed with ultramarine blue, however, both pigments lose their unpleasant qualities of color and become a very deep, pure color, which is most brilliant if the colors are glazed one over the other alternately. Prussian blue being cheap is in every ordinary paint box. It is useless for murals, as it is destroyed immediately by lime.
COBALT BLUE is an expensive color. It is imitated in ordinary paint boxes by a mixture of ultramarine and zinc white, which never achieves the exceptional clarity of real cobalt. It is clear even when applied thickly. It is a pigment to be used by itself, as it is easily swamped by other colors when mixed; the best mixture is with viridian. It is not very strong in tinting power, and stands midway between a glaze and an opaque color. It looks very rich and has a fascinating effect, particularly in fresco but also in watercolor and mat tempera. Some of its charm is lost when it is mixed with a fat, shiny oil binder.
Small additions of cobalt blue make oil colors dry more quickly. It can be obtained in two tones, and there are two exceptionally attractive variants as well: cerulean blue, the color of a cloudless sky and very valuable for atmospheric tones, and blue-green oxide (cobalt tin color), often used to reproduce the color of the winter sky close to the horizon. There is no sense in making any mixtures with these two. They must stand alone, like jewels.
Both these variants of cobalt are equally unaffected by lime and resemble cobalt in all other technical and coloring qualities, including resistance to heat. They are used for porcelain painting and pot glazes.