This is for the PC, however it should be the same for the other systems
The one with the purple smoke is the one you want to drink (the smoke will disappear once it's done moving), find it at the start, and you have to remember which one it is as it changes spots, I'm pretty sure they move at random (90% sure), so I can't tell you the order, (I'd trace where it was with my finger).
Hope this helps, good luck (you'll like the ending)