First, this function won't update when formatting is changed.  Formatting
doesn't cause excel to recalc.

You could make the function volatile:
Public Function Color(ByVal rgeCell As Range) As Integer
  application.volatile
  Color = rgeCell.Interior.ColorIndex
End Function

But this still could be one calculation behind--leading to incorrect results.

Second, this function returns the colorindex of the cell when you apply the
formatting--not the color from conditional formatting.

If you want to try to return the color based on conditional formatting, you
could review the code from Chip Pearson's site:

http://cpearson.com/excel/CFColors.htm

I think it's far from trivial.  You may want to use another cell that mimics the
same conditions, but returns a number.  It may be lots easier.

Excel doesn't recognize a change of color as an Event.  Events are what
causes the spreadsheet to get updated and causes macros to run.  Therefore,
colors cannot cause any updates of calculations nor can it cause a macro to
run .