I saw the documentary too and couldn't help noticing that Terry's wig has moved with the times. Back in 1968 when he hosted the aptly named Rose of Tralee contest the one he was wearing was as black as a raven although something about the way it was parted made it look a bit stiff and lifeless.....
I wonder if he started off with a grey wig and dyed it black at first gradually reducing the intensity by adding the odd sprinkle of grey as time moved on?