this.richTextBox1.SelectionColor = Color.Red
this.richTextBox1.text += "Test"

richTextBox1.Text += "Test"
richTextBox1.Select(richTextBox1.TextLength - 4, 4)
richTextBox1.SelectionColor = Color.Red

this.richTextBox1.SelectionColor = Color.Red

this.richTextBox1.text="Hello world!"
this.richTextBox1.selstart=0
this.richTextBox1.sellength=5
this.richTextBox1.SelectionColor = Color.Red

RichTextBox1.SelectionColor = Color.Red
RichTextBox1.SelectedText = "Hello "
RichTextBox1.SelectionColor = Color.Green
RichTextBox1.SelectedText = "World"