using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace gitCompare
{
        public partial class Form1 : Form
        {
                public Form1()
                {
                        InitializeComponent();
                }

                private void Form1_Load(object sender, EventArgs e)
                {
                        this.richTextBox1.Text += "This is some text. ";
                        this.richTextBox1.SelectionColor = Color.CadetBlue;
                        this.richTextBox1.Text += "This is some text. ";

                        richTextBox1.SelectionStart = 5;
                        richTextBox1.SelectionLength = 10;
                        richTextBox1.SelectionColor = Color.CadetBlue;

                        this.resetCursorSelectionColor();
                }

                private void button1_Click(object sender, EventArgs e)
                {
                        richTextBox1.Text += textBox1.Text;
                        richTextBox1.SelectionStart = richTextBox1.Text.Length 
                                - textBox1.Text.Length;
                        richTextBox1.SelectionLength = textBox1.Text.Length;
                        richTextBox1.SelectionColor = Color.Crimson;

                        this.resetCursorSelectionColor();
                }

                public void resetCursorSelectionColor()
                {
                        richTextBox1.SelectionStart = richTextBox1.Text.Length;
                        richTextBox1.SelectionColor = Color.Black;      
                        // still writes crimson after click.

                        richTextBox1.SelectionStart = 0;
                        richTextBox1.SelectionLength = 0;
                        richTextBox1.SelectionColor = Color.Black;

                }

        }
}

Labels: c#, code