Can photopolymer resin colors be mixed?
Yes and No at the same time
First of all, yes, you can mix resins. However, you should only mix resins that are of the same makeup, as in one brand and type. Why? because different types of resin have different compositions and different polymerization types. Mixing different types can result in unpredictable behavior, and not working at all!
To test, use a tiny amount of your brand and type matched resins and apply to the vat, then print something really tiny, like just 2 or 3 layers of a 20x20x20 mm cube.