Colour Plates/Engravings
A duo-tone is made from a black and white copy using the same negative and halftone screen angle for both plates but the second plate is etched differently. Other duo-tone effects may be achieved by using one plate only and printing the second colour slightly out of register.
Two Colour Process
These plates are produced by using a black and white copy but making two negatives with different screen angles, 45 for the dark colour and 75 for the light.
Three & Four Colour Process
These are produced from a colour copy using colour filters and different screen angles for each plate. The filters used are coloured glass or gelatine film and are placed in front of the camera lens during the making of the negatives
The filters used are violet (for yellow), green (for red) orange/red (for blue) The violet filter excludes all the yellow values from affecting the negative thus giving the yellow positive or yellow printing plate. The same principle applies when making the remaining colour plates. To stop the various colours from printing one on top of each other dot for dot (which would give a muddy black print) varying screen angles are used. These are: blue 45, red 75 and yellow 105.

3 colour plate

4 colour plate

It is not suggested that blue must be 105 or red 75 or vice versa as naturally these colour angles can be varied. These angles enable accurate colour values to be achieved and also stop a moire pattern or distortion.

Screens and their use
Process engraving screens for the letterpress process are made with so many lines (#) per inch, depending on the quality of reproduction required. The following table outlines their use.

             175# : For special scientific and medical textbooks. Finest art paper.
             150# : For finest art paper.
             133# : For art and imitation art.
             120# : For imitation art papers.
             100# : For super calendar (SC) and imitation art.
               85# : For machine finish (MF) or super calendered papers (SC).
               65# : For fine newsprint.
               50# : For coarse newsprint.

Screens of 200 and 225 can also be obtained but are very rarely used. Duplicate plates (electrotype or stereotypes) should not be made from such fine screened plates.