In the recent times, I've seen lots of results in Google and here about LED streetlighting turning strange color and lots of Americans calling them "Purple" or even seeing them as violet or purple instead of blue.
Here are my few answers what happening here:
1. All common white LEDs are consisting of a blue emitter and a phosphor that converts it to white. Although white LED with violet emitters are exists, they are extremely rare and expensive, and white LEDs with UV emitters are yet to be developed for commercial application with high lumen efficacy and low cost.
2. Unlike regular streetlighting, which are different in design between regions (Americans had cobraheads, UK had Thorn and GEC, European have quadrangular streetlights without prismatic bowls and etc...), LED streetlights are the same in all regions.
3. In the case of the American LED streetlights, most of them losing their phosphor to expose the base color of the emitter which should be blue. Despite this, in most pictures, the color seen as purple or violet instead of blue, and these are also the names that the Americans calling them.
4. As the wavelength of most blue emitters of white LEDs is very short, most digital cameras and displays capturing it as purple or violet. This is also the reason why the broadcasts describing the color as purple, because TV cameras and press cameras sees deep blue as violet or purple as well.
5. It seems to me that some people sees deep blue as purple or violet as a result of optical illusion. This is the same case of people seeing the color of SOX/LPS lamps as yellow, despite it is amber actually.

When I first saw pictures of these malfunctioning LEDs, I thought that something in the circuitry had failed causing the lights to go into some kind of UV or blacklight mode. It would be interesting to see how one of these malfunctioning lights compares to an actual LED blacklight. I'd say the wavelength of the malfunctioning light would be pretty close to an LED blacklight

You can analyze the spectrum of one of these to see what cause of this color, because I doubt these lanterns don't have blue emitter.

When I first saw pictures of these malfunctioning LEDs, I thought that something in the circuitry had failed causing the lights to go into some kind of UV or blacklight mode. It would be interesting to see how one of these malfunctioning lights compares to an actual LED blacklight. I'd say the wavelength of the malfunctioning light would be pretty close to an LED blacklight

Unlike discharges, white LED's start from a blue emitting chip, with the phosphors then absorbing some of that and converting it to the rest. Generally two major phosphor components are used: Onwe radiating in the yellow/green and the second, used with higher CRI LED's, in orange/red (both tend to have rather wide spectrum, not the spikes normally known from the CRI80 fluorescent lamps). The ratio of these two components and their general coverage (so the amount of the primary blue radiation they absorb) is then varied to get the desired output color parameters.
Here it seems the phosphor component responsible for the green/yellow part of the spectrum has degraded (or degraded more than the reddish component), so the LED continued to emit just with the blue primary and then the red component, yielding the purplish result.

In theory if the components were on separate LED dies (to allow more advanced control,...), the greenish section may have failed electrically. But I wouldn't expect this to by systematic among many fixtures, or at least there would be as many "purplish", as the "greenish" (when the reddish section failes) failures.

Because it seems the "purplish" failure mode is virtually the only one and I wouldn't expect the two section type used with streetlights (is more for products with user variable CCT), I would guess it is most likely the degradation of the greenish phosphor component.

Some LED streetlights use UV LED chips instead of blue ones, usually to get better color rendering by eliminating the “blue spike” that LEDs are infamous for.

These types of white LEDs, has yet went into commercial production, because of lower lumen efficiency than blue LED + phosphor.

The technical color name of the blue LEDs is called “royal blue”.

Yeah so, not really. The reason people describe it as purple is because they really do light up purple. My job involves driving around at night looking for lights that are out and fixing them. During this, I see a lot of them turning purple. I haven't seen a single one that is just blue. They all have a bit of purple. It's not just the cameras. While the base emitter is indeed blue, the reason they light up purple is because the phosphor pads fall off, but still remain on the lenses. So you have the blue light from the diodes passing through, and along the phosphor pad. Those produce the other colors which then mix with the blue, creating purple, as Medved explained. It's no misnomer. It's real life. I see it at work all the time.

It would be interesting to see how one of these malfunctioning lights compares to an actual LED blacklight.

I put a B/W portable CRT television underneath one, and the phosphor screen didn't light up. But my jacket with neon orange did.

I believe the phosphor that is excited by blue light, where the "Excitation Wavelength" is in the 450-470 nm range (strictly in the blue) MUST be made from a Yttrium Aluminum Garnet (YAG) material. Any other material will not fluoresce.

So to make it clear to me, there are no violet diode based street-lights with 3 phosphors out on North American roads??

Would love to obtain the spectrum of one of these. How can I get a hold of one of these "purple" lights or streetlight? Personally I have never seen one, nor do I know someone near me that has seen one. Alternately, how can I cause an LED to go "purple"??

You can analyze the spectrum of one of these to see what cause of this color, because I doubt these lanterns don't have blue emitter.

there is a full works yard near my work full of AEL Autobahns that have turned "purple"
and there are 2 at stages of blue-ing I pass every night and am "watching" them and so far in 2 months since I noticed them they have NOT degraded MORE YET