The colors that make a person up could be shifted to a part of
   light that goes BEYOND the rainbow. Lower than Red or higher
   than Violet. Everything could work exactly as it would for a
   visible person, but humans would be unable to see that person at
   all. You would not see a shadow because the invisible person
   would not affect ANY of the rainbow colors, and shadows that
   humans see are only made when rainbow colors are blocked. If you
   don't block rainbow colors, you don't cast shadows. But would an
   invisible person be able to see another invisible person? Maybe:
   if their eyes could also see a bigger rainbow than we can see.