roll random:  gain resistance to acid damage in an encounter with a Red Dragon.  PC: *FML*

Roll if you want a random energy type; otherwise the DM would be setting an appropriate type of the potion.

The entry doesn't provide a description of the potion's appearance, so I use the following description based on the one for Potion of Invulnerability.

"This potion is a thick, syrupy liquid of a color appropriate to the type of resistance it provides."

If in doubt, use a color for a dragon with a breath weapon that deals that type of damage.

I think of the the resistances as falling naturally into two types, standard dragon breaths (acid, cold, fire, lightning, and poison), which would be slightly more common in my game, and other damage types (force, necrotic, psychic, radiant, and thunder), which would be slightly more rare.

Thus, when finding loot in the remains of a study belonging to a wizard who studied dragons, I rewarded my players with a small case containing five vials of potion of resistance - one for each type of dragon breath.

You come across a burned out house. Inside resting on the floor are 3 Potions of Fire Resistance. What happened here?

Great reason for why the BBEG is also resisting my warlock's EB  :)

This potion is basically useless, because the damage type is determined randomly when you DRINK it. So you cannot use it strategically at all.

Quote from IgnavumFortunaRepugnat >>

This potion is basically useless, because the damage type is determined randomly when you DRINK it. So you cannot use it strategically at all.

 I disagree. It doesn't say the potion is determined when you drink it, just that the DM chooses the type or determines it randomly by rolling on the table. 

I'd interpret is as the potion is determined randomly when you find it or identify it.