Sometimes the vials add or remove skill when the villager "doesn't feel like themselves" -- so you have to know their skills to figure out something happened.

I've never seen them change appearance, so that's pretty cool (you can also change their appearance with the F6 key, I believe -- search the forums and you'll find some discussions about it).

I don't know if there's a relationship between the color of the liquid and the effect -- I didn't bother to keep track of what liquids my villagers were drinking .