C&M states dragons are immune to all natural elements.
I know that, I talk about how we use that information, i.e. the books talks about magical resistance, but there is no resistance number in any place, so anyone must use its own ones.
And, in the case of elemental resistance, the reasonable point for me is that they are immune to their own element, and resistant to others (by their skins), but not immune. Has it much sense to think a fire dragon immune to cold?, IMO, no.
It says
natural, and not, magical attacks are not like natural ones, you can see the elemental resistance spells, they make cater immune to natual elements but only apply an OB penalty against magical attacks.
But, in any case, I think immunity and resistance should be used, i.e. a fire dragon usually sleeps in hot places, like a volcano, why not they sleep in an ice cave?, wait, that is place for ice dragons. It has sense, not?. Immune means it sense nothing from it, so why dragons don't sleep in any place and they have prefered sites for it?.