This is air code, I didn't test it. You'll need to specify what color you want the non-filled cells to be.
Dim rng As Excel.Range
Dim cell As Excel.Range
Set rng = Selection
Application.ScreenUpdating = False
For Each cell In rng
If cell.Interior.ColorIndex = -4142 Then ' no fill color
' what color do you want?
' cell.Interior.ColorIndex = ???
Application.ScreenUpdating = True