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

Perhaps my question is too simple as I cannot find an appropriate answer in the forums or online references.

I need to interactively format many iterated worksheets that are generated with an external program. Once done, I need to add new columns and format those columns. There is much repetition in the columns (comparisons against an index sheet) and repeating all that formatting code is unwieldy and I suspect slowing down performance. So to get to it:

Sub MainOne()
Dim ColumnVar as Range

ColumnVar = "C:C"

FormatItNow(columnVar)

<<rest of code>>

End Sub


Sub FormatItNow(ColumnVar)
Range(ColumnVar).Select

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

End Sub

This pseudocode example would give me an error (type mismatch) every time. Futzing with quotes did no good.

Any help on the correct syntax would be very much appreciated.

Thank you in advance.