using System;
using System.Collections.Generic;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace DocDetector.Core.Extensions
{
 public static class RichTextBoxExtension
 {
  public static void AppendTextColorful(this RichTextBox rtBox, string text, Color color, bool addNewLine = true)
  {
   if (addNewLine)
   {
    text += Environment.NewLine;
   }
   rtBox.SelectionStart = rtBox.TextLength;
   rtBox.SelectionLength = 0;
   rtBox.SelectionColor = color;
   rtBox.AppendText(text);
   rtBox.SelectionColor = rtBox.ForeColor;
  }
 }
}

rtxtLog.AppendTextColorful("Your message here",Color.Green);