Concatenate only where there is data

  Dim Cll As Range
  Dim Group1Color As Long
  Dim Group2Color As Long
  Dim Group As Integer
  Dim lrColB As Long
  
  lrColB = Range("B" & Cells.Count).End(xlUp).Row
  
  Group1Color = -4142 'No Fill
  Group2Color = 15    'Gray
  Group = 1
  
  Range("C1:C" & lrColB).Insert Shift:=xlToRight, CopyOrigin:=xlFormatFromLeftOrAbove
  'snip...

Thank you very much for the information Kenneth. When I ran the updated macro, I received an "Overflow" message. There were 2 places where I incorporated the "& lrColB". Did I mess up my updates? Here is the updated code:

 Sub group_and_shade()
'
' group_and_shade Macro
'

'
Dim Cll As Range
    Dim Group1Color As Long
    Dim Group2Color As Long
    Dim Group As Integer
    Dim lrColB As Long
    
    lrColB = Range("B" & Cells.Count).End(x1Up).Row
    
    Group1Color = -4142 'No Fill
    Group2Color = 15    'Gray
    Group = 1

    Range("C1:C" & lrColB).Select
    Selection.Insert Shift:=xlToRight, CopyOrigin:=xlFormatFromLeftOrAbove
    Range("C1").Select
    ActiveCell.FormulaR1C1 = "=CONCATENATE(RC[-2],RC[-1])"
    Selection.Copy
    Range("C1:C" & lrColB).Select
    ActiveSheet.Paste
    Application.CutCopyMode = False
    Selection.Copy
    Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks _
        :=False, Transpose:=False
    Application.CutCopyMode = False
    For Each Cll In Range("C2:C" & Range("C:C").SpecialCells(xlCellTypeLastCell).Row)
        If Cll.Offset(-1, 0).Value <> Cll.Value Then
            Group = Group * -1
        End If
        If Group > 0 Then
            Cll.EntireRow.Interior.ColorIndex = Group1Color
        Else
            Cll.EntireRow.Interior.ColorIndex = Group2Color
        End If
    Next Cll
    Selection.Delete Shift:=xlToLeft
End Sub

Sorry, forgot the rows part.

    lrColB = Range("B" & Cells.Rows.Count).End(xlUp).Row

I don't use the SpecialCells method because it only updates after a save. Use a method like mine.

Originally Posted by Kenneth Hobson

I don't use the SpecialCells method because it only updates after a save. Use a method like mine.

Oops. Didn't read fully before posting. I didn't realize that specialcells use the last saved values. Thanks.

I get a "400" error code when I run the macro now. I have trimmed the code to this, and still get the "400" error code:

 Sub group_and_shade()
'
' group_and_shade Macro
'

'
Dim lrColB As Long
    
    lrColB = Range("B" & Cells.Rows.Count).End(x1Up).Row
    
    Range("C1:C" & lrColB).Select
    Selection.Copy
    
End Sub

Still getting the "400" error message

I like to use this method:

Activesheet.UsedRange.SpecialCells(xlLastCell).Row

Or

Range("B:B").SpecialCells(xlLastCell).Row

There were 2 things going on:
1. My blindness, and
2. The error code 400 will appear if there is no data on the spreadsheet. When I was troubleshooting, I kept moving components of code to new spreadsheets. Didn't think to take the data as well.

Thank you very much everyone!