I want to create an On/Off button that will temporarily show which cells
contain formulas.

If someone knows a better way pls say so.

I thought of adding and then subtracting a fixed number from the
Interior.color property but I don't know how to retrieve the current value.

rngFormulas.Select
If flag = False Then
With Selection.Interior
.ColorIndex = ????? + 300000
End With
flag = True
Else
With Selection.Interior
.ColorIndex = ????? - 300000
End With
flag = False
End If