I'm going to cast my own bathroom countertop in concrete. We are trying to match the floor tiles in black level, so I did three samples, each with a double amount of pigment than the last. Of course it didn't match.
The big big one is the target. You can use white pigment if you need to make it more less dark.
Of course a picture will never have the exact same shade as reality, but the difference in black level should be pretty accurate (since the 3 samples are from the same picture). So what I did was to use Gaussian blur on each color and this is what I got:
I would like to somehow get the black level of each sample, and calculate an average. Also, the difference between the tile and the lightest sample. I can use that to get the amount of pigment.