I'm honestly not sure how Vanilla handle that with potion, but maybe try register your own ItemColors, then you can give different tint index to the different cubes, or more specifically, faces. Those values will be given when asking you to calculate your own colour. For example, I have north tint index 3, south index 2, then I will have something like:
return tintIndex == 2 ? red : blue;
or something like that, depends on what you want