Code:

  Private Sub cmdTest_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdTest.Click
    Dim Knop As Control
    Dim KnopNaam As String = "Button"

    For Each Knop In Me.Controls
      If TypeOf Knop Is Button Then
        If Knop.Name.Substring(0, KnopNaam.Length) = KnopNaam Then
          Select Case Knop.Tag.ToString
            Case "1", "3"
              Knop.ForeColor = Color.Red
            Case "2", "4"
              Knop.ForeColor = Color.Blue
          End Select
        End If
      End If
    Next
  End Sub