"It wasn’t me. It was my wand. My wand acted of its own accord. The bike was falling. I couldn’t have told you where Voldemort was, but my wand spun in my hand and found him and shot a spell at him, and it wasn’t even a spell I recognized. I’ve never made gold flames appear before."
-- Harry Potter (DH5)
Golden fire is the description of an unnamed, mysterious, and extremely powerful jet of golden fire between two wands. The effect occurred in the midst of the Battle of the Seven Potters when Harry’s wand, on its own volition, turned in his hand and attacked Voldemort with a powerful blast of flames. According to Ollivander, a wand firing golden flames on its own was something completely unknown to him:
“I had . . . never heard of such a thing. Your wand performed something unique that night. The connection of the twin cores is incredibly rare, yet why your wand would have snapped the borrowed wand, I do not know. . . .”
— Ollivander (DH24)