adecualo a tu codigo 

Código (vb) [Seleccionar]
Private Sub Form_Load()
Dim pos As Long
RichTextBox1.Text = "Sergio dice: Hola" & vbCrLf & _
"Sergio dice: Hola" & vbCrLf & _
"Sergio dice: Hola" & vbCrLf & _
"Sergio dice: Hola"
pos = 1
Do While InStr(pos, RichTextBox1.Text, ":") <> 0
RichTextBox1.SelStart = InStr(pos, RichTextBox1.Text, ":") + 1
RichTextBox1.SelLength = 4
RichTextBox1.SelBold = True
RichTextBox1.SelColor = vbBlue
RichTextBox1.SelStart = Len(RichTextBox1.Text)
pos = InStr(pos, RichTextBox1.Text, ":") + 1
Loop
End Sub