Hi Ellen,
A friend of mine has a similar coffee table, except instead of a blue towel
she actually has sand in it!  As far as fading from light is concerned, it is
ultraviolet radiation that is primarily responsible for the destruction of
pigments.  Direct sunlight is the most harmful, followed by indirect
sunlight.  Ordinary room lights won't cause much fading, if any.  Also,
glass, especially the heavy glass used in coffee table tops, filters out a
lot of UV, so your shells are probably pretty safe.
Paul M.