When different colors are to be superimposed, the order in which they are printed is a matter of some importance. As a rule, the so called warm tones (yellow, brown, red, or colors containing much of these) are printed first; the cold tones (blue or others containing much blue) being afterwards laid on. White is also classed as a cold tone, but for technical reasons it must nearly always be printed first. If these rules are' observed, serious mistakes will seldom be made.
In order that the colors may appear as bright as possible, a little varnish is added to them, which in every case must be of a kind to suit the particular color. Dammar varnish is suitable for light colors and for darker colors amber varnish. Where great brilliancy is required, Venetian turpentine is added; but this lengthens the time required for drying if a little too much is added.

In every case, however, a little siccative must be added to every color a few drops materially quicken the drying.
Color blocking can only be done when the blocks have been secured to the plate, as this must be opened each time it is colored.

The simplest color printing is where one color appears between the impression in gold, and in this case it is generally done by first making the impression in gold and then in color.

In more complex work several colors are to be used; at the present time the work is frequently produced requiring colors side by side and superimposed, together with gold, bronze, and aluminum.

The process is in each case as follows: From the block supplied by the engraver an impression is made which forms a matrix. It is best to select a piece of board exactly the size of the covers to be printed and at the two places to be taken by the guide points pieces of vellum are firmly pasted, being alre:1dy pierced for the guide points, these vellum additions being further secured by pasting over them a strip of paper or cloth. For every block to be used similar board with its vellum additions is prepared. Upon the ground of this first matrix the other auxiliary matrices are likewise first impressed with the block-all of them black, of course, so that it would be possible at any time to fit in the blocks.

Every new block for color printing is pressed into the ground of the auxiliary matrix belonging to it; also for the so called first impression block.

It must be understood that color printing can be done well on]y upon a perfectly even and smooth surface, therefore it is wise with first-impression blocks, as well as larger color plates, to repeat the impression, only slightly warm after having given a very hot impression the first time.

Surface papers are an exception as they generally stand very little heat. It helps here either to let the newly made covers get half dry or, better, to let them lie longer in a damp room.

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