In this video at 12:27, when asked about picks, Collen's Guitar Tech John Zocco stated:

[Thomann]: "And the picks, I've heard some rumors about metal picks!"

[Zocco]: "Yes, they're brass, super thick I mean there's no give to these things at all..."

[T]: "Does he always use the brass picks?"

[JZ]: "It was steel up until a few years ago, and then somebody gave him a brass one, just a random, generic brass one from somewhere, that he loved and wound up using it the entire tour, this is right before I started working for him. And he had to kinda protect that pick cause it was like, the only one he had, so he'd like, put it in his pocket after a show, and just you know, really baby it cause that was the only one so... And after that Dunlop started making him the brass ones with his signature, and the Leppard logo, and all that stuff, it goes hand-in-hand with the 13s and everything just being big & heavy you know? and even with these he never breaks strings, (and now that I said that he'll probably start so...)"

[T]: "It makes sence because if you have like thicker strings, they will last way longer anyhow, then you can kinda tourture them with some materials that are not nessarly most string-friendly ones, but its probably going to work as long as normal picks would work with 9s or 10s"

[JZ]: "Yeah, and he likes the sound of it, its got a unique sound you know, when he's picking fast with it you can really hear the attack, the brass just kinda imparts its own tonality to it aswell, even other picks, I mean, I exparament with that all the time, between Tortex and Ultex, everything sounds a little bit different, and sometimes you don't think that about the tonal aspect of a pick, you just think how it feels, but the tone, it really changes the tone."

In a separate article Collen himself stated:

[PC]: "My buddy Rudy, who I grew up playing guitar with, in London, we've always had this quest for tone and sounds and Floyd Rose, DiMarzio, all that stuff, so we're constantly doing these, 'Hey, check this out...'

"And I played this thing, this brass pick, and he went 'Oh my god, this is the best pick!' So he gave it to me, I used it for the whole British tour.

"So, if I dropped it, I'd be looking on the floor. So the last gig is in London, it's Wembley, Brian May's there, from Queen...

"And I said, 'Brian, I've got this thing, I've got this brass pick that my buddy Rudy gave to me. I play this because it's kind of a tribute to you. You use the sixpence, so here you go.'

"He said, 'Oh, no, no, I can't if a friend gave it to you.' I said 'No, I'm sure he'd be fine if Brian May's got the pick...'

"So yeah, Brian took it, and since then Dunlop makes me brass picks with Def Leppard logo. It's got my name on there, it's got 'Def Leppard.'"