There are instruments to measure color, spectrophotometers, that would provide a variety of data to measure the changes in color due to fading. You could also use a simpler instrument, like a photographic light meter, to measure the change in "lightness" of the paper. Even with the measurements there will be issues of interpretation. For example a 2% change in black paper might look far more objectionable than a 2% change in white paper. The PANTONE system is really a color naming system, so it doesn't provide a systematic way to measure changes and compare them across different colors. It might be best to do the evaluation visually and simply have some observers rank order the changes from smallest to largest after various periods of exposure to sunlight. As far as preserving the colors with sunlight exposure, it is possible that some type of varnish or clear coating would help but it is hard to say and would require some experimentation. Even a piece of glass might help (e.g.