I cannot confirm, but a Formlabs employee advised me that the photoinitiators are in the pigment, which is why a precise 100 mL of pigment should be added to the base. If correct, adding a non-Formlabs pigment would likely result in the resin not curing properly.
You may could start with the Formlabs pigments and then add something to that to reach the desired color. But if any experiments fail, then you’ve wasted a bunch of material/money.