ColorSniffer.cs

using System;
using System.Drawing;
using System.Threading;
using System.Runtime.InteropServices;

namespace ConsoleApp1
{
    class Program
    {
        [DllImport("user32.dll", SetLastError = true)]
        public static extern IntPtr GetDesktopWindow();
        [DllImport("user32.dll", SetLastError = true)]
        public static extern IntPtr GetWindowDC(IntPtr window);
        [DllImport("gdi32.dll", SetLastError = true)]
        public static extern uint GetPixel(IntPtr dc, int x, int y);
        [DllImport("user32.dll", SetLastError = true)]
        public static extern int ReleaseDC(IntPtr window, IntPtr dc);

        private static bool contact, lastContact;
        private static float hue, lastHue;

        static void Main(string[] args)
        {
            Timer t = new Timer(TimerCallback, null, 0, 250);
            Console.ReadLine();
        }

        private static void TimerCallback(Object o)
        {
            Color c = GetColorAt(3000, 500);
            hue = c.GetHue();

            if (Math.Abs(hue - lastHue) >= 20)
                contact = true;
            else
                contact = false;

            lastHue = hue;

            if (contact != lastContact)
            {
                Console.WriteLine("Contact " + contact.ToString());
                lastContact = contact;
            }

            GC.Collect();
        }

        public static Color GetColorAt(int x, int y)
        {
            IntPtr desk = GetDesktopWindow();
            IntPtr dc = GetWindowDC(desk);
            int a = (int)GetPixel(dc, x, y);
            ReleaseDC(desk, dc);
            return Color.FromArgb(255, (a >> 0) & 0xff, (a >> 8) & 0xff, (a >> 16) & 0xff);
        }
    }
}