“Please be careful, Hagrid. I’m still not sure this was advisable and it’s certainly only to be used in emergencies.”
-- Arthur Weasley, talking about the purple button (DH4)
Dragonfire is a blue-white gout of flame which is emitted with a characteristic roar from exhaust of the flying motorbike when the purple button next to the speedometer. The dragonfire device was installed by Arthur Weasley prior to Hagrid using the bike to rescue Harry from Privet Drive (DH4).