e_ are two pigments, one very light and
clear, the other darker, which are made of the oxide of the metal
cobalt. In oil they are permanent, and do not change when mixed with
other colors. For delicate tints, when the tones are to be subtly gray
yet full of the primary colors, the cobalts are indispensable. You
should always have them on hand, and generally on your palette.
Cerulean blue is of less importance than the other, but in very clear,
delicate blue skies it is often the only color which will get the
effect.

_Prussian blue_ possesses a depth and power and a quality of color
which make it unique. The greenish tone gives it great value in
certain combinations _as far as its tinting effect is concerned_. But
it is not reliable as a pigment. It changes under various conditions,
and fades with the light. It is not to be depended upon. _Antwerp
blue_, a weaker kind of Prussian blue, is even more fugitive. It is a
pity that these colors will not stand, but as they will not, we must
get along without them.

_Indigo_ has a certain grayish quality which is useful sometimes, but
it cannot be placed among the even moderately permanent colors.

_The blacks_ may be classed as blues, because they will make green if
mixed with yellow. Considered as blues, they are, of course, dense and
negative, and should not be too freely used. But they are all
permanent. The only ones we need speak of are _ivory black_, which has
a reddish cast, and _blue black_, which is weaker, but lacks the
purplish note, which is often an advantage.

=Greens.=--We need mention only a few greens. There are numerous
greens, of various degrees of permanence, but it is not necessary to
speak of all the colors on the market. You could not use them all if
you had them, and we may as well confine ourselves to those we really
need.

_Veridian_, or _emeraude green_, is the deepest and coldest of our
greens, and is permanent. It is too cold, and looks even more so at
night. In use it needs the addition of some yellow which holds its own
at night, such as yellow ochre, or the painting will be impossible in
gaslight, and even worse under electric light.

_Emerald green_ is the same as the French _Veronese_ green, and is
generally permanent. It is said to turn dark, and does lose some of
its brilliancy with time and the effect of impure air. But there are
places where one needs it, especially in sketching, and it is well to
use it sometimes. But bear in mind