AV, pg 121 - Storm Shield daily power:
"Power (Daily ✦ Lightning, Thunder): Immediate Reaction.
Use this power when you are hit by a melee attack.
Deal 2d6 lightning and thunder damage to the attacker.
(The attacker must have resistance or immunity to both
damage types to reduce or ignore this damage.)"
Since it's written in parenthesis I believe it's reminder text, which should indicate a general rule and not a specific exception.
Since it contradicts the general rule, I'd say it's not reminder text, given there's NO RULE IN EXISTANCE TO REMIND YOU OF. This is not Magic: The Gathering. -That- game has the rule where text in perentheses is reminder text. This game is not Magic: The Gathering. Applying Magic: The Gathering rules to Dungeons and Dragons makes as much sense as applying Dungeons and Dragons rules to Monopoly. (which could be awesome) I've seen people confusing 'resistance' with 'immunity', it should not surprise me that there would be confusion between 'Magic: The Gathering' and 'Dungeons and Dragons.'
Anyways:
Using the PHB 1 rules to adjudicate it (and no other book) means that if you have immunity to something, and you get hit with a power with that keyword, nada happens. No damage occurs. This is because the PHB1 rule is that
foo effects do not affect someone who is immune to
foo. And while 25 fire and cold damage is both fire and cold damage, if the power in question has the Fire keyword, then all effects of that power
including damage are Fire effects, and therefore immunity to fire is a blanket NO to all of it.
That means Dire Radiance doesn't deal damage to creatures immune to fear.
And so on.
This is why we do not apply only the PHB1 wording.
That is not the rule you apply anymore, as it is superceded by the PHB3 rule which is more exact about what immunity actually means.
Now, in the case of damage with multiple types:
Resistance or immunity to one keyword of a power does not protect a target from the power’s other effects.
That's the ONLY part of the rule on multiple damage types that applies to or even MENTIONS immunity. Then you follow immunity.
15 cold and fire damage is fire damage. Immunity says you are not affected by fire damage. It is not resistance infinity. It is 'The Damage Does Not Happen.'
Therefore we search for any rule that says that damage with multiple types is not affected by immunity to only one.
THIS EXCEPTION DOES NOT EXIST.
In exception-based rules systems, when you have a general rule, if an exception exists, you follow the exception.
The corrolary of that is, if the exception does not exist, you DO NOT FOLLOW THE NON-EXISTANT EXCEPTION.
Also:
Vulnerability is -still- not 'negative resistance' and doesn't behave as resistance does. It hasn't been for the entirety of the existance of fourth edition. Immunity has never been resistance, in ANY edition. This isn't difficult to grok.
A is not B.
Stop trying to apply the rules for resistance involving multi-typed damage to every other rule in the game.