Light fastness in NuPastel
TL;DR

About a quarter to a third of the pastel colours visibly change when exposed to natural light for a period of several months.
Among pastels (of any brand), reds and pinks seem to suffer the most from light.
In reaction to a test (referenced in this answer), NuPastel removed the 17 least lightfast colours from their product range somewhere in or before 2019.
No official lightfastness testing has been carried out by NuPastel, since they're (still) not legally obliged to.



Back in 2013, a 90 day light-fastness test of 95 specimens of NuPastels was performed by Rick Petersen, and the results were posted on his blog.
Petersen placed a sheet with streaks of the 95 different colours in a south-facing window (from January to April, in what I think is Florida, U.S.A.), meaning they received a lot of direct sunlight.
The results were judged by eye after 90 days.
The colours that showed the most change (&quot;significant change in hue and value&quot;) were Pale Vermillion, Salmon Pink, Orchid Pink, and Old Lilac; the outlier being Madder Pink which seems to have almost completely faded.
Orange, Maroon, Burnt Orange, Carmine, Rose Pink, Carnival Red, and Flamingo also didn't fare well, showing &quot;slight change in hue and value&quot;.
&quot;No changes&quot; were observed in most ochres and other earth tones, the greens, browns, and yellows, however. This is likely due to them containing iron hydroxides, which &quot;are the most stable lightfast materials in any media&quot; (source).
66 of the 95 colours passed this 3-month test without visible changes.

Another test of the PrismaColors NuPastels was performed by Colleen Wampole in 2015, over a period of six months. Her conclusion was as follows:

Some colors within the Prismacolor Nupastel line faded dramatically during this six-month period. This was expected, since this lightfastness test was inspired by my observations of these pastels fading within the store at which I work. Pastels have a reputation for being fugitive, and this line of pastels was no exception.
The colors which faded the most were: B[urnt] Orange, Deep Orange, Madder Pink, Salmon Pink, Pale Vermilion, Scarlet, Persian Rose, Orchid, Eden Green, Carnival Pink, Orchid Pink, Rose Pink, Crimson, Old Lilac, Maroon, Pistachio Green, and Neptune Green.
I noticed slight fading in the following colors: Light Naples Hue, Chrome Yellow, Burgundy, Light Sap Green, Carmine Madder, Flamingo, and Carmine Red.

(The results can be seen here (unfortunately these photos there are of very low quality).*)
71 of the 95 colours stood this 6-month test.
It's interesting to see how a bunch of these colours overlaps with Petersen's results, but others acted completely differently: Eden Green, Pistachio Green, and Neptune Green, specifically. Do these react differently to other environmental conditions, or is their reaction to UV non-linear?



Comments posted by customers who bought the NuPastels and left a review, support these findings or speak of similar conclusions.  
Even though there are relatively few users mentioning the light-fastness, do note how most of these comments are based on hear-say or personal tests of which the results are inaccessible.

A 2-star rating on DickBlick mentions &quot;I have done my own tests and have found that the majority of the Nupastel colors fade. I encourage anyone who doubts this to do their own tests and see for themselves&quot;.
A 3-star review tells how &quot;the company has told [the user] they do no testing on their pastels for lightfastness as it's not required.&quot;

On WetCanvas user PeggyB opines &quot;Are they using NuPastels as their “underlayer”? Now there is a brand I’d question the quality of! There are way too many of them that are known to be not lightfast.&quot;


More interestingly:

In 2019, a customer review on Amazon mentions how, based on Petersen's test referred to above, they &quot;removed the 17 colors in the 2-5 categories&quot;.


However, while this reflects badly on PrismaColor NuPastels, the problematic lightfastness of certain colours is not unique to specific brands. Disappointing as it is, a lack of lightfastness testing also seems to be the norm.
Someone on WetCanvas states the following in this thread:

I’ve come across Jackson’s house brand of soft pastels. They have the pigments listed and lightfast ratings for each stick. Though I’ve been confused and wonder if there are mistakes in the information they have listed. They’re listed as Excellent, Good or Fair, similar to their watercolor and gouache ratings. I understand what that means at least in the water media, and am not sure if it is the same for pastels, though I would imagine so. I haven’t gone through every color for their pastels, but so many of the pinks, lilac, violets, Carmine’s, magenta’s are considered fair, which at least for watercolors means not lightfast enough I would use it for any serious works. I understand these are always the problem colors in terms of lightfastness.

A lightfastness test of almost 5 years of Charvin Paris pastel sticks here shows a remarkably similar result. In her summary, Jamie Williams Grossman writes:

The good news is that the earth colors and grays did not change. [..] The yellow ochre is beautiful, and held up perfectly.
Only one blue showed significant change, in spite of the fact that most of the blues are rated only &quot;***Good.&quot;
Three of the greens I felt did well enough to hang onto. The others either lost much or all of their yellow component, or in one case lost blue.
Now for the really bad news. All of the pinks, reds and violets have got to go. The only bright yellow in the set is also completely fugitive, and nearly disappeared like a magic trick. (I'm sure it would be gone completely if left a few more years in the window!) Most of the yellow tints also faded.

The results can be seen here: yellows, reds and pinks, greens, greys, browns, and blues.*
All of the findings and conclusions mentioned here are repeated in this post.

An interesting read is the following comment written by Michael Skalka, chairman of the subcommittee of the American Society of Testing and Materials (ASTM) D01.57, Artists' Materials, in reply to Katherine Tyrrell, which was published on the latter's website. She had contacted him after making the claim in a WetCanvas thread that no ASTM standard exists for pastel lightfastness.

No lightfastness standard from ASTM exists yet. Manufacturers, not consumers, have been the driving force behind getting oil paints, watercolors, gouache, acrylics to have lightfastness standards. Standards create a level playing field and that brings out the best from all concerned. Companies that can’t conform stand out when a standard can be used as a yardstick to determine overall quality.
Nobody has the edge on lightfastness for pastels yet because no standard exists. You can argue that some manufacturers knowingly put fugitive materials in pastels but without them conducting a well -formulated testing protocol, they cannot know if the colors they use are lightfast. So for right now ignorance by the manufacturers is bliss.


* I've uploaded the images of the specific tests on Imgur for longevity's sake. The credits and rights of these are for their specific authors, of course.