The problem is as follows:
I have a few objects that are using the same material. I want each to have different color (texture modulated by an RGB value) and different opacity.
There's a "Modulate color" value in a Sprite3D node that allows to make per-object changes regardless of the material, but this doesn't work if I use a non-standard material.
How do I modify objects' color and alpha without changing the look of all other objects using the same material?