You are holding a green mortar filled with pitch-black sap in your right hand and a vial filled with a swirling purple juice in your left hand.
alchemy mix vial with mortar
You cannot mix the purple vial with anything.
I then tried just shaking the vial, and I got the swirling purple potion. I remembered having, for some reason I can't recall, poured the contents of that particular vial out. Does reusing a vial prevent mixing with the contents of a mortar? This is how I have made the white potions every time.