Something similar happened in our long-lasting Pathfinder campaign in Mystara.
Our brave adventurers heard rumors of a great dragon terrorizing a place, and as heroes do they set out to defeat the evil! None of the witnesses could confirm the color of the dragon (it was dark and the dragon seemed kind of dark? they were mostly "omg a dragon!!!! panic!!!" themselves so they didn't stop to look closely).
So we head out to where the lair is rumored to be, and lo and behold, a Black Dragon appears! Casters get Resist Energy up, the Magus starts to prep up his best Electricity stuff, and as we engage the Dragon in melee, the Magus unleashed his Intensified Maximized Shocking Grasp on the Dragon... with a Critical Hit to boot! And look, he got through Spell Resistance too!
Cue the haughty laugh of the Dragon as he reveals himself to be a Blue Dragon in disguise (Illusion spells are great).