Purple and Blue Fireworks!

So, I see a lot of fireworks. The Washington Nationals' home stadium is visible from my balcony, and they have fireworks after home games, plus on and around July 4 locals set off illegal-without-a-license fireworks in the stadium parking lot by unwritten agreement with the police.* There's also the SWDC Spring Festival, with its fireworks show off a barge in the Potomac; I can see both the rehearsal and the show from my balcony, in the opposite direction to the stadium. And of course there's July 4.

I mean, I'm not an expert by any means, it's not like I'm seeing shows every weekend, and I generally don't see shows outside this area. But still, I see significantly more fireworks than most people's 1-2 times a year.

Anyway, the reason I bring this up is I've noticed an interesting trend over the last couple of years: a steady increase in the number of royal blue** fireworks I see, from few to none up until 2011 or so, to them being quite common now. (I think they're more common than green or bright blue at this point.) And then over the last year, I've been noticing a surge in the number of bright purple fireworks.

Is this a local thing? Just a shift in fads? Or is there some underlying reason? Have the metallic salts used to make those shades dropped in price recently, or has somebody come up with a new technique for making them?

I tried googling for answers and came up with nothing. I must say, I quite like them, though! But still, the finale over the stadium last week, ending a show dominated by cold colors with a warm explosion of yellows and oranges, was quite nice as a change of pace.

*The police know that people are going to drive out to Pennsylvania or West Virginia and buy fireworks (both states have laws restricting setting off fireworks and restricting the sale of fireworks to residents of the state, but no laws restricting the sale of fireworks to visitors from out of state). They also know that it is vastly safer if those fireworks are set off in the biggest stretch of concrete in the city, the stadium parking lot, then in crowded neighborhoods with lots of flammable materials around. So they quietly refrain from enforcing fireworks laws after hours in that parking lot.

**I think they're royal blue, anyway. My color vision is slightly wonky. They look sort of dark purple to me, and everyone else calls them blue. Usually that means royal blue, just as if something looks black to me and everyone else calls it blue it's navy blue. I have yet to figure out if the shades of brown that look black to me have a name.