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°.