Apparently doing a lightfastness test is now controversial? People will really try to create clicks out of anything. No I won’t link to the guy ranting about artists doing their own lightfastness tests and calling out manufacturers for their BS. Instead I’ll like to this video by Sarah Renae Clark, who does a good job explaining the blue wool test, why it’s kind of garbage for art supplies, and how she did her tests.
I am currently using a North West facing window that gets full sun from about noon to sunset- during a Massachusetts summer. Roughly 8 to 9 hours a day from July to August. Yes that’s a lot of sun, yes it gets hot in the room and on the pane of glass. My goal isn’t to create museum or typical hanging situations, it’s to see how long a material will last when applied the way I use it. This means I’m not applying it in the way the manufacturer applied it in their tests. I’m doing washes, I’m scribbling, I’m swatching.
Anyway, I feel comfortable that my lightfastness tests give me the information I need to make art that lasts. You should conduct your own or look for results from other artists who make art like yours who share their work.
I’m also going to add here, I think lightfastness is important if you sell your work. It is less important if you are only working in a journal for yourself. I have art that I made that has been on my wall for 20 years made from who knows what kind of materials and it has probably faded more than I know. And yeah I hang my own art on the wall.
Now let’s discuss these tests I did with my current urban sketching/en plein air supplies.
I have been pleasantly surprised by the Arrtx watercolors, Beam Paints, Art of Soil, Meeden soft bodied acrylics and Stockmar Crayons. Almost all the colors showed little to now shifting except for the Arrtx Cad Red Pale and 2 of the shades of green in the Meeden soft bodied paints. None of the Beam or Art of Soil paints shifted at all. The Stockmar Crayons in bright bold primary and secondary colors are aimed at children and I expected them to respond like other brands of crayons and disappear, but they did not. Instead the colors are bright, bold, and vibrant- and the window didn’t get hot enough to melt the wax into the paper*.

Some disappointing results- Arteeza gouache. More of the colors disappeared or shifted than I expected. The “nude” colored Miya charcoal pencil shifted drastically. A number of the Aliexpress woodless charcoal pencils also disappeared or shifted drastically in color. 
The most disappointing result was the number of Prismacolor colored pencils- both regular and watercolor that changed color or disappeared. It was just really disheartening to use a supposedly professional (Premier) colored pencil and have the color just absolutely change or disappear.

I wasn’t at all surprised when the Seami watercolors completely disappeared. This was pretty much an expected result. 
*I am just going to write this here, I don’t think my window gets hot enough to cook any of the materials placed in it. The wax would have heated and melted into the paper if that were the case. There is no wax halo and the texture of the wax is still felt on the surface of the smooth Strathmore mixed media paper. So do with that what you will.
** IMO Lightfastness tests by individual artists are important because they give you information on how the material will last in the way that an artist uses it. You know I’m not the person suggesting that the only good materials are lightfast materials. I use more lightfast materials on works that I’m going to sell. I use anything I want in my sketches and art journal. You do you. That said, I do think that manufacturers should list accurate information on their tools. There have been a number of artists doing lightfastness tests on materials coming from budget friendly brands. Lightwish, Phoenix, Rubens, Mart Marie, etc… And more than a few of the items claims don’t match up with the artist tests. Here are some of the videos that offer up real world bright light tests over time:
https://youtu.be/plfnSqz67bo?si=fCQH_OxZZjvfiGgA
https://youtu.be/GlmOHZMlKus?si=Bwwq0Mc9SSQcIObs
















These are Mungyo gallery oil pastels. They are the only pastels in my air tight container that got harder over time. they were also much more firm than other brands.
Mont Marie. Soft silky smooth, even after over a year of storage. Also, available in a tin and not quite as smooth as Rubens or Lightwish.





