Basically, if you had iron or metal type armor equipped you’d take damage at the same speed but the damage is higher if it’s hit by a heat kind of magic
Pretend your holding a metal/pan but its on fire, the metal/iron would be hotter so if heat magics touched metal/iron armor then it should do more damage if wearing, but if this is added, this also effects the iron magic, if a fire user hits a iron user it would do slightly more damage but the iron would be stronger because of the heat.
May I ask a little question? What would be the reason to add this? Would it be realism or something else since no clear reason has been stated for this to be put into consideration.
Plasma and fire can already heat up iron spells, which I believe increases the damage (not sure about the effect).
It’s also not very clear if you want iron users themselve to be vulnerable to fire, but assuming you do, it doesn’t make sense since you arent even made of iron or anything and it would be unbalanced anyway