The hydra description says, "To prevent a severed head from growing back into two heads, at least 5 points of fire or acid damage must be dealt to the stump (a touch attack to hit) before the new heads appear" (Monster Manual 155–6). The pyrohydra description adds, "Fire attacks cannot prevent a pyrohydra’s stump from growing new heads…, but 5 points of cold damage does" (157)
The creature I'm customizing for a challenging upcoming encounter is a half-white dragon 5-headed pyrohydra. Although consumed with self-loathing, the hydra is immune to fire damage from the subtype fire and immune to cold damage from the template half-white dragon.
A close reading seems to indicate that this creature's stumps can still be prevented from growing heads if a stump—rather than the creature proper—is dealt 5 or more points of cold damage. That is, damage to the stump seems to be tracked separately from damage to the hydra's heads or body.
Is it accurate to rule that this creature's stumps can still be cauterized by cold damage? Or am I reading this description too deeply, and this half-white dragon pyrohydra's stumps are just as resilient to cold damage as the rest of it?