Try this
vb Code:
  1. Private Sub AddText(ByVal Text As String, ByVal Color As Drawing.Color)
  2.         Dim strWords() As String = Text.Split(" "c)
  3.         Dim colors(3) As Drawing.Color
  4.  
  5.         colors(0) = Drawing.Color.Red
  6.         colors(1) = Drawing.Color.Blue
  7.         colors(2) = Drawing.Color.Green
  8.         colors(3) = Drawing.Color.Brown
  9.         ' add more colors if you need
  10.  
  11.         With txtChat
  12.             For j As Integer = 0 To strWords.Length - 1
  13.                 .SelectionColor = colors(j Mod colors.Length)
  14.                 .AppendText(strWords(j) & " ")
  15.             Next
  16.         End With
  17.  
  18.     End Sub