The stamps in this set were printed using a combination of 4 colours ore more, with each colour being used for a different part of the design. The colours were each printed separately by passing the sheets through the presses multiple times. Most of the time the colours are pure colours, but occasionally the colour you see results from the mixing of two or more colours. I have found variations in each component colour, and often the variations will be found in many permutations and combinations. Some differences are so dramatic, it is hard to believe that Scott or Gibbons does not list them separately.
Below are some images of the variation found on each value:
I have not illustrated the 7k, 8k, 15k, 18k, 25k, 30k, 35k, 50k or 1N values, but these exhibit many shades as well, although they tend to be more subtle than say the 5k or 10k values.