I assume your 3rd grader is running this experiment on the Science Buddies Website: http://www.sciencebuddies.org/science-f ... p084.shtml
, or one very similar to it.
In the experiment as described on the Science Buddies site, the experimenter is instructed to use colored markers to draw on a clear plastic sheet. If one holds that sheet up to the light, they see the colors. That is because the dye in each marker allows only a small range of light wavelength to pass through. So you are correct in suspecting that wavelength of light is involved.
So by placing this clear sheet on a piece of sun-print paper, each marker stripe allows different wavelength ranges of light to pass through to the paper. The experimenter is then instructed to determine which colors of light, that is which wavelengths of light, result in better images. This allows one to draw the conclusion what colors of light is the sun-print paper most sensitive to.
I hope this helps.
I'm glad you are working with your student on this project. Parent involvement is so important. I hope you HAVE FUN!