Syntax for visible range between two columns to last row

Since you did not show your entire code I will assume there is a With statement indicating the sheet reference. With a little analysis we can observe that your case is 1 to 12 and that your column is sequential and 7 more than MFRmo. The red line that you indicated is flawed in many ways so I rewrote what I think you are trying to accomplish

MFRmo = Wb.Sheets("Start Tab").Range("G2").Value 'what's our MFR month?

        Select Case MFRmo
            Case 1 To 12
                Result = MFRmo + 7 'this will give a numerical value which will accurately represent the column number
        End Select
        
        'This section is okay.  I would like to see the range defined a little bit more.
        Set RngFound = .Cells.Find(What:="Projected")
        If Not RngFound Is Nothing Then
            Proj = RngFound.Column
        End If

        'Color our expense headings Peach
        .Range("A1").AutoFilter Field:=3, Criteria1:="=Category"
        .Range("H1:" & Result & LastRow).SpecialCells(xlCellTypeVisible).Interior.Color = RGB(252, 213, 180)
Stop
        'Color our projection headings Blue
        '.Range(.Range(Result & Rows(1)).Offset(0, 1), .Columns(Proj)).SpecialCells(xlCellTypeVisible).Interior.Color = RGB(183, 222, 232)
        .Range(.Cells(1, Result), .Cells(.Cells(Rows.Count, Proj).End(xlUp).Row, Proj)).SpecialCells(xlCellTypeVisible).Interior.Color = RGB(183, 222, 232)
        
        'Take off the autofilter
        .AutoFilterMode = False