Fortunately both options are in the game on at least one NPC: the Vargul Blighthound in Zul'Drak has red flames and the Hungering Plaguehound in Icecrown has pinker flames. I haven't been able to check Grimkor's Hound personally, as it's a quest spawn for a quest chain I've already done and I'll need more time to check it. But from Wowhead's info I'm pretty sure it's the pink one too.
Screenshots (updated for clearer images):
Red Flames:
Pink Flames: