Subject: Why does the spectrum change when viewed through a water prism?

Why does the spectrum change when viewed through a water prism?
I set up a water prism in my class, and put various black and white
cards on a stand about 5 feet away on a long table.  When viewing the
spectrum, on top of white it appears as yellow through red, but when
looking above black it appears violet through blue.  I cannot find out
the scientific reason for this difference.  Can you help?  I teach
gifted students, and this is from a fifth grade optics unit.

Thank you!
Jill Dembsky

Re:Why does the spectrum change when viewed through a water prism?