Just as a test, I edited the doRange code just to see which cells were
identified as rngTopLeft. In column F, beginning at F7, every other
cell blue, instead of the top left of every block (i.e., F7, F9,
F11.....F47). The same goes for Column M. The code I entered is as
follows:

With rngTopLeft
.Interior.ColorIndex = 37
End With

End Sub