The theory is pretty straightforward: find all pixels within a specified tolerance around the overprint color. For stamps with the high difference between the base and the overprint color this will work just fine. However, in practice you will get stamps where the base and the overprint color overlaps to a certain extent.
The base stamp with black overprint and its extraction with 50% and a 20% tolerances.
In the case above, you can see that the black color is contained in the base stamp too, so the difference between the colors is not sufficiently large to extract the overprint easily. At 50% tolerance a lot of the base stamp remains while at 20% tolerance (where the base stamp disappears) a lot of the overprint is missing.
The solution is therefore in finding ways how we can increase the color difference. One way is to experiment with the hue and saturation.
The base stamp with different hue and saturation changes. The last one is the most suitable.
Thus, we can get an image with greater color contrast and the extraction of the overprint becomes relatively trivial.
Extraction of black overprint with 20% tolerance.
