I made a model for potions and used the white stained glass texture for the bottle part and an opaque "potion overlay" texture on the inside of the bottle.

The problem is that, even though the overlay is opaque and the stained glass is translucent, the overlay cannot be seen from the outside.

You can see as I go close up to it, the overlay is definitely working properly with a potion of poison used as the example below.

[EDIT 1]: Short video of the problem: http://i.gyazo.com/96ef13fdfd943706f1801e7943d238cf.mp4