Color code analog from VB6 to VB.NET

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VB6

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Hello, im new at vb.net, what im trying to do is that textbox3 and texbox5 have a data, and i want to make a "compare" button, when i click it, it will collor they data diffrences with red, colored data will be shown in texbox3. the code from vb6(this codes works well btw):

Private Sub Command5_Click()
Dim a As Integer
Dim R1 As String
Dim r2 As String
Dim strimm As String
strimm = RichTextBox1.Text
 RichTextBox1 = Replace(strimm, vbCrLf, "")

strimm = RichTextBox2.Text
 RichTextBox2 = Replace(strimm, vbCrLf, "")
For a = 0 To Len(RichTextBox2)

RichTextBox1.SelStart = a
RichTextBox1.SelLength = 1
R1 = RichTextBox1.SelText
RichTextBox2.SelStart = a
RichTextBox2.SelLength = 1
r2 = RichTextBox2.SelText
If R1 <> r2 Then
RichTextBox1.SelColor = vbRed
End If
Next
End Sub

What I have tried:

The code i tried in VB.net( it colores whole data with red, if only 1 is not eqaul, thats an error, i need to only diffrence data colored):

Private Sub Command5_Click(sender As Object, e As EventArgs) Handles Command5.Click

        Dim a As Integer
        Dim R1 As String
        Dim r2 As String
        Dim strimm As String
        strimm = TextBox3.Text
        TextBox3.Text = Replace(strimm, vbCrLf, "")

        strimm = TextBox5.Text
        TextBox5.Text = Replace(strimm, vbCrLf, "")
        For a = 0 To TextBox5.TextLength


            TextBox3.SelectionStart = a
            TextBox3.SelectionLength = 1
            R1 = TextBox3.SelectedText
            TextBox5.SelectionStart = a
            TextBox5.SelectionLength = 1
            r2 = TextBox5.SelectedText
            If R1 <> r2 Then
                TextBox3.ForeColor = Color.Red
            End If
        Next
    End Sub