c# richtextbox修改字体大小和颜色

一开始：
this.richTextBox1.Text = "欢迎";
            this.richTextBox1.SelectAll();
            this.richTextBox1.SelectionColor = Color.Red;
            Font font = new Font(FontFamily.GenericMonospace,14,FontStyle.Regular);       
            this.richTextBox1.SelectionFont = font;

点击按钮：
this.richTextBox1.Text = "谢谢";
            this.richTextBox1.SelectAll();
            this.richTextBox1.SelectionColor = Color.Black;
            Font font = new Font(FontFamily.GenericMonospace,10,FontStyle.Regular);       
            this.richTextBox1.SelectionFont = font;