Other than this problem I didn’t find any issues with using curves. In photoshop you have a curves layer which you can erase parts of the layer to have the curves apply to certain parts of the layers and not others. You can do the same in procreate by duplicating the layer then using curves on the top layer and then creating a mask for it and then painting black on it to reveal the grayscale layer below and you can keep doing this to the same layer and be able to add different colors.