How do you pass a column range as an argument to a subroutine?

Try:

Sub MainOne()
Dim ColumnVar As Range

Set ColumnVar = Range("C:C")

Call FormatItNow(ColumnVar)

'<<rest of code>>

End Sub


Sub FormatItNow(ColumnVar As Range)

With ColumnVar.FormatConditions(1).Interior
.PatternColorIndex = xlAutomatic
.Color = 13551615
.TintAndShade = 0
End With
'etc

End Sub