Translucency
If there is a monochrome translucent shape overlayed on a background, this tool can be used to calculate the transparency of that shape as well as its colour. You must supply two samples, each of which has the original colour as well as the colour when the translucent shape is on top.
α:
Result:
Demonstration and details

Here I’ve overlayed a translucent blue rectangle on orange and purple.
Note that the 0.554 corresponds to the 54% opacity used. There are three different α values because it’s calculated from all three R, G and B channels – in theory, they should all be the same, but we take the average of the three anyway.
It’s possible to get values of α less than 0 or greater than 1. If this happens, something has probably gone wrong, and you should re-check whether you are in fact looking at something translucent.