On a few occasions I have been told emphatically by students and colleagues that Prang watercolors, an elementary school staple, are lightfast except for the yellow (according to one professional user). I have nothing against Prang paints, and for $10 they are, in my view, a bargain for the 16 color set. For youth classes and for workshop attendees who don't want to invest in artist quality paints, they will do just fine. But lightfast? Some of the colors appear to be more stable than others, but 9 of the 16 colors in my set purchased in 2014 degraded significantly if not fatally after 7 months of light exposure:

The color samples were created on November 20, 2014. One set was placed on an east-facing windowsill in my attached sun room. The control set was kept out of direct light in my studio.

In row one, the only colors that did not degrade are the three on the right: ultramarine blue, brown, and black. The three warm colors on the left faded considerably as can be clearly seen.

In row two, even the white essentially vanished after 7.5 months. The magenta turned pale blue. The orange turned pale yellow except where a concentration of paint puddled. Or, more likely, perhaps the row one color strip lay across a corner of the row 2 strip to protect the orange and the lower half of the red. 

I can still recommend the Prang watercolors for exercises done in workshops or for children. But for work that you want to preserve, invest in a small pan set or tubes of high quality paints.