change into rainbow color for every single characters (tested on Microsoft Word 2016)

color.bas

Sub color()
  Application.ScreenUpdating = False
  Dim i As Long
  Dim color
  'rainbow color
  color = Array(8388736, 255, 33023, 65535, 65280, 16776960, 16711680)
  With ActiveDocument
    For i = 1 To .Characters.Count
      .Characters(i).Font.color = color(i Mod 7)
    Next
  End With
  Application.ScreenUpdating = True
End Sub