How To Do a Simple Lightfastness Test for a Paint Color

Here's How:

1. Paint a swatch or area of the color(s) you want to test for lightfastness on a sheet of heavy, smooth watercolor paper. If you're testing multiple colors, leave a section of white paper between them. Either apply the paint as you would usually use it in a painting or, be more thorough and paint a range, from thin to totally opaque.
2. Paint the samples large enough and in such a way that you'll easily be able to cut it in half. Write the name of each color next to the sample.
3. When you've painted all the colors you want to test for lightfastness, cut the sheet in half. Place one half in a dark drawer; this will be used as the "before exposure to sunlight" comparison.
4. Place the other half of the sheet in bright, direct sunlight. Stick it to the inside of a window that gets a lot of sun or if your car is parked in the sun, on the dashboard (provided the glass isn't UV-protective!).
5. Now you have to be patient and let the sun do its thing. After a month, but ideally several more, compare the half of the sheet kept in the shade to the one in the sunlight. Any differences indicate that a color is not lightfast.

Tips:

1. Check that the sheet of watercolor paper doesn't have a yellowish bias to it, especially if you're doing a lightfastness test with watercolor paints.
2. If this simple lightfastness test grabs your interest, you might want to move on to do a comprehensive lightfastness test.