What does rain bow spell damage do?

I can only find it on 1 weapon, forest aconite.

 

Idk what it does differently from raw spell, now that raw spell is divided up into the elemental damage of your weapon.

But it's just that, raw spell damage though you need to powder Forest Aconite before it actually applies that damage, though it will only add damage to the element type of the powder, so it isn't actually rainbow?