The power armour in Fallout 4 has a headlight by default on the helmet and you can replace it with different colour headlights or a brighter/stronger one. But how do you turn it on? I've searched everywhere and tried every button.
On the PC version you hold down the TAB key - for Playstation 4 you hold the O button and for Xbox you hold the B button.
This is the same button as to bring up the Pip Boy (although in that instance you press rather than hold). This is also the same key binding to enable the flashlight when on foot and the flashlight on the mining helmet.