I don't believe there's any way to get a cell's color from a formula. The closest you can get is the CELL
formula, but (at least as of Excel 2003), it doesn't return the cell's color.
It would be pretty easy to implement with VBA:
Public Function myColor(r As Range) As Integer
myColor = r.Interior.ColorIndex
End Function
Then in the worksheet:
=mycolor(A1)