Some colors fly or fade very quickly, while others are perfectly permanent. The following is a list of the principal permanent colors under ordinary conditions: Yellow ochre, light red, Indian red, umber, cadmium yellow, Prussian blue, cobalt blue, Vandyke brown, red ochre, sienna, red oxide, Venetian red, vermilion, ultramarine, chrome green, lamp black, and other black pigments.

Probably the simplest method of testing the durability of colors is to provide a sheet of unglazed cardboard; that known as Bristol board will do very well. It must have so slight an absorbent property that if any coat of paint is placed on the surface it will remain there, and not soak into the substance of the cardboard. This sheet of board is ruled into squares or rectangles measuring about 3x3 inches, or 2x2 inches.

A little of the color to be tested is ground up with a little gum water into a smooth paste, and a portion of one of the ruled spaces on the cardboard painted with it. It is advisable to rule and prepare two sheets at the same time. The name of the color can be written either underneath the patch of color in the square, or in a corresponding position on the back of the card. It is also advisable to grind a little of the pigment with oil, so that the relative durability as a water color and as an oil color can be tested.

One of the prepared cards is hung in a place where it is exposed to as much sunlight and air as possible, while the other card is placed in a drawer away from any such influence. After a week or two of exposure the cards can be compared to see if any changes have occurred; they can then be replaced in their respective positions, and from time to time are compared together. Any change which may have been brought about by the action of sunlight and air on the exposed card will be observable; some colors will be changed in a few weeks' exposure, other colors require months of exposure to produce any effect.

By placing a card painted in the manner described, with different pigments in a closed cupboard, in which is placed a vessel containing some ferrous sulphide and diluted sulphuric acid, the action of sulphuretted hydrogen on the colors can be tested; if any are affected by this test it is certain that they will be similarly affected when exposed to the action of impure air.