MainWindow.xaml.cs

using System.Windows;

namespace WpfRichTextBox
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            new System.Windows.Documents.TextRange(customRichTextBox.Document.ContentStart, customRichTextBox.Document.ContentEnd).Text = "Hello World!";
            if (customRichTextBox.TextLines.Count > 0 && customRichTextBox.TextLines[0].RichChars.Count > 0)
            {
                customRichTextBox.TextLines[0].RichChars[0].Color = System.Windows.Media.Colors.Red;
                customRichTextBox.TextLines[0].RichChars[0].Position = new System.Windows.Point(50, 50);
                customRichTextBox.InvalidateVisual();
            }
        }
    }
}