Updated: Sep 21, 2022
UV exposure is damaging to all works of art. The same pigments make up the base for all artist's mediums (oil, acrylic, watercolor, pastel, and others). Over time the colors of some pigments can fade or even completely change when exposed to light. Museums are full of historic paintings by masters such as Van Gogh, Renoir, Munch, Matisse, Turner and many more that have faded or changed over time.
Some colors are more prone to fading in sunlight than others. When a color fades or changes very easily its what's known as a "fugitive color". Older/Vintage pastels may have fugitive colors because some of the pigments they're made from weren't light-fast. Newer pastels typically have less problems with fugitive colors because modern pigments have been formulated to replaced older pigments with known light-fastness issues. That being said newer pastels aren't guaranteed to be light-fast, some colors are just naturally more prone to fading than others. Student-grade pastels can sometimes contain inferior pigments that might fade more readily when exposed to light. So no matter the age of your pastels, it's worth doing a test to be sure they won't fade or change easily when exposed to sunlight.
To check for light-fastness, you can do a simple sunlight test with two sample sheets.
To make sample sheets get two pieces of pastel paper and the colors you'd like to test. Put a swatch of color on each sheet, it's important to place the colors in the same place on each piece so you can easily compare them later. If you'd like, you can also label the color swatches to help remember the colors being tested. When you're done you should have two identical sets of color swatches on each sheet.
Cover one sheet with something thick enough to block out all sunlight (e.g. illustration board or card stock). Place the other sheet in a sunny south facing window, leave it uncovered and note the date you placed it there.
After a month check to see if there are any differences by comparing the two sheets. Note color changes (if any) and place the sheet back in the window.
Check every month for three to six months (three in summer, six in winter). After this initial test period if you don't have any dramatic changes, it's probably safe to say that you don't have any really bad fugitive colors. If you do see color changes, remove those pastels from your collection, with longer sunlight exposure the changes will only get worse and you don't want those fugitive colors changing in your painting the way they changed on your windowsill!
To be really thorough, you can keep the test going for a year (or more), checking the sheet for changes every few months.
All colors, no matter their light-fastness, can eventually experience some changes due to light exposure. That's why it's important to frame original artwork under glass that has UV light protection. It's also a good idea to display original artwork in a spot that's away from direct sunlight.