Hello LarryC,

Here is the revised macro. When using event macros that change worksheet values be sure to disable further events and re-enable them when you code has finished.
Private Sub Worksheet_SelectionChange(ByVal Target As Range)

  Dim cell As Range
  Dim formulacells As Range
  
  Const REDINDEX = 3
  Const GREENINDEX = 4

    Application.ScreenUpdating = False
    Application.EnableEvents = False

    On Error Resume Next
     'create subsets of original selection
      Set formulacells = Selection.SpecialCells(xlFormulas, xlNumbers)
        If Err.Number <> 0 Then Err.Clear
    On Error GoTo 0

     'Process the formula cells
      If Not formulacells Is Nothing Then
        For Each cell In formulacells
          If cell.Value = 1 Then
             cell.Interior.ColorIndex = GREENINDEX
          ElseIf cell.Value < 1 Then
             cell.Interior.ColorIndex = REDINDEX
          Else
            cell.Interior.ColorIndex = xlNone
          End If
        Next cell
      End If

    Application.EnableEvents = True

End Sub
Sincerely,
Leith Ross