If a Barbarian drank this and then raged would they o my take 1/4th melee damage or 1/3rd? Or something completely different 

When a barbarian rages they gain resistance to bludgeoning, piercing, and slashing damage. This item gives them resistance to all damage. Those two things don't stack, so the barbarian would still take 1/2. It wouldn't change because resistance doesn't stack.

Bloodborne's Lead Elixir?

Give this to your lich bbeg to use during their monolog

Does anyone know whether or not this would be distinguishable from actual liquid iron?

Liquid Iron or molten iron is normal redish.
this would probably be more like mercury, but has the color of iron

Invulnerability has a meaning both in D&D and IRL. Yet, this potion does not confer those either of those definitions. Instead it offers Resistance. Imagine if a Potion of Flying just let you jump really far. That's this potion.

Quote from NotActuallyKyle >>

Invulnerability has a meaning both in D&D and IRL. Yet, this potion does not confer those either of those definitions. Instead it offers Resistance. Imagine if a Potion of Flying just let you jump really far. That's this potion.

 good point

Quote from Hogan_Nantor >>

If a Barbarian drank this and then raged would they o my take 1/4th melee damage or 1/3rd? Or something completely different 

 Resistance does not stack.  You either have it or you don't.

What if you mix it with another potion and get a 00 on the dice