这是最有效的：它在光标所在的位置抓取一个像素，并且不依赖于只有一个监视器。

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.Runtime.InteropServices;
using System.Windows.Forms;
using System.Diagnostics;

namespace FormTest
{
    public partial class Form1 : Form
    {
        [DllImport("user32.dll")]
        static extern bool GetCursorPos(ref Point lpPoint);

        [DllImport("gdi32.dll", CharSet = CharSet.Auto, SetLastError = true, ExactSpelling = true)]
        public static extern int BitBlt(IntPtr hDC, int x, int y, int nWidth, int nHeight, IntPtr hSrcDC, int xSrc, int ySrc, int dwRop);

        public Form1()
        {
            InitializeComponent();
        }

        private void MouseMoveTimer_Tick(object sender, EventArgs e)
        {
            Point cursor = new Point();
            GetCursorPos(ref cursor);

            var c = GetColorAt(cursor);
            this.BackColor = c;

            if (c.R == c.G && c.G < 64 && c.B > 128)
            {
                MessageBox.Show("Blue");
            }
        }

        Bitmap screenPixel = new Bitmap(1, 1, PixelFormat.Format32bppArgb);
        public Color GetColorAt(Point location)
        {
            using (Graphics gdest = Graphics.FromImage(screenPixel))
            {
                using (Graphics gsrc = Graphics.FromHwnd(IntPtr.Zero))
                {
                    IntPtr hSrcDC = gsrc.GetHdc();
                    IntPtr hDC = gdest.GetHdc();
                    int retval = BitBlt(hDC, 0, 0, 1, 1, hSrcDC, location.X, location.Y, (int)CopyPixelOperation.SourceCopy);
                    gdest.ReleaseHdc();
                    gsrc.ReleaseHdc();
                }
            }

            return screenPixel.GetPixel(0, 0);
        }
    }
}

现在，显然，您不必使用光标的当前位置，但这是一般的想法。

编辑：

给定上面的GetColorAt函数，你可以用一种安全、性能友好的方式轮询屏幕上的某个像素，如下所示：

private void PollPixel(Point location, Color color)
{
    while(true)
    {
        var c = GetColorAt(location);

        if (c.R == color.R && c.G == color.G && c.B == color.B)
        {
            DoAction();
            return;
        }

        // By calling Thread.Sleep() without a parameter, we are signaling to the
        // operating system that we only want to sleep long enough for other
        // applications.  As soon as the other apps yield their CPU time, we will
        // regain control.
        Thread.Sleep()
    }
}

如果需要，您可以将其 Package 在Thread中，或者从Console应用程序中执行它。

这里的大多数答案都使用了该像素的相同来源（桌面dc）。
键功能为GetPixel。

[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);

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);
}

我想这是最干净最快捷的方式。

备注：

如果您在Windows上修改了“显示设置”中的默认文本大小，以提高高分辨率显示器的可读性，则需要以相同的方式调整GetPixel（）的坐标参数。例如，如果在Windows 7上光标位置为（x，y），文本大小为150%，则需要调用GetPixel（x1.5，y1.5）来获取光标下像素的颜色。

此函数更短，使用System.Drawing也能达到相同的结果，但不使用Pinvoke。

Color GetColorAt(int x, int y)
{
    Bitmap bmp = new Bitmap(1, 1);
    Rectangle bounds = new Rectangle(x, y, 1, 1);
    using (Graphics g = Graphics.FromImage(bmp))
        g.CopyFromScreen(bounds.Location, Point.Empty, bounds.Size);
    return bmp.GetPixel(0, 0);
}

请检查这两个不同的功能，我用在我以前的项目之一：
1)此函数可拍摄桌面的快照

private void CaptureScreenAndSave(string strSavePath)
        {

            //SetTitle("Capturing Screen...");

            Bitmap bmpScreenshot;

            Graphics gfxScreenshot;
            bmpScreenshot = new Bitmap(Screen.PrimaryScreen.Bounds.Width, Screen.PrimaryScreen.Bounds.Height,System.Drawing.Imaging.PixelFormat.Format32bppArgb);
            gfxScreenshot = Graphics.FromImage(bmpScreenshot);
            gfxScreenshot.CopyFromScreen(Screen.PrimaryScreen.Bounds.X, Screen.PrimaryScreen.Bounds.Y, 0, 0, Screen.PrimaryScreen.Bounds.Size, CopyPixelOperation.SourceCopy);
            MemoryStream msIn = new MemoryStream();
            bmpScreenshot.Save(msIn, System.Drawing.Imaging.ImageCodecInfo.GetImageEncoders()[0], null);

            msIn.Close();

            byte[] buf = msIn.ToArray();

            MemoryStream msOut = new MemoryStream();

            msOut.Write(buf, 0, buf.Length);

            msOut.Position = 0;

            Bitmap bmpOut = new Bitmap(msOut);

            try
            {
                bmpOut.Save(strSavePath, System.Drawing.Imaging.ImageFormat.Bmp);
                //SetTitle("Capturing Screen Image Saved...");
            }

            catch (Exception exp)
            {

            }

            finally
            {
                msOut.Close();
            }
        }

2)此函数采用输入图像，并计算给定像素范围的RGB平均值。

double GetRGBAverageForPixelRange( int istartRange, int iEndRange,  Bitmap oBitmap )
        {
            double dRetnVal = 0 ;
            Color oTempColor ; 
            int i, j ;
            for( int iCounter = istartRange ; iCounter < iEndRange ; iCounter++ )
            {
                i = (iCounter % (oBitmap.Width));
                j = ( iCounter / ( oBitmap.Width ) ) ;
                if (i >= 0 && j >= 0 && i < oBitmap.Width && j < oBitmap.Height )
                {
                    oTempColor = oBitmap.GetPixel(i, j);
                    dRetnVal = dRetnVal + oTempColor.ToArgb();
                }

            }
            return dRetnVal ;
        }

这两个函数一起可能会解决你的问题。快乐编码：）

EDIT：请注意，GetPixel是一个非常慢的函数。在使用它之前，我会三思而后行。

据我所知最简单的办法是：

1. take a screenshot
   1.查看位图并获取像素颜色

编辑

可能没有办法“等待”直到像素变成某种颜色。你的程序可能不得不经常循环检查它，直到它看到颜色。
例如：

while(!IsPixelColor(x, y, color))
{
    //probably best to add a sleep here so your program doesn't use too much CPU
}
DoAction();

编辑2

下面是一些可以修改的示例代码。这段代码只是根据给定像素中的当前颜色更改标签的颜色。这段代码避免了上面提到的句柄泄漏。

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

namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{

    [DllImport("gdi32.dll", CharSet = CharSet.Auto, SetLastError = true, ExactSpelling = true)]
    public static extern int BitBlt(IntPtr hDC, int x, int y, int nWidth, int nHeight, IntPtr hSrcDC, int xSrc, int ySrc, int dwRop);

    Thread t;
    int x, y;

    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        x = 20;
        y = 50;
        t = new Thread(update);
        t.Start();
    }

    private void update()
    {
        Bitmap screenCopy = new Bitmap(1, 1);
        using (Graphics gdest = Graphics.FromImage(screenCopy))
        {
            while (true)
            {
                //g.CopyFromScreen(new Point(0, 0), new Point(0, 0), new Size(256, 256));
                using (Graphics gsrc = Graphics.FromHwnd(IntPtr.Zero))
                {
                    IntPtr hSrcDC = gsrc.GetHdc();
                    IntPtr hDC = gdest.GetHdc();
                    int retval = BitBlt(hDC, 0, 0, 1, 1, hSrcDC, x, y, (int)CopyPixelOperation.SourceCopy);
                    gdest.ReleaseHdc();
                    gsrc.ReleaseHdc();
                }
                Color c = Color.FromArgb(screenCopy.GetPixel(0, 0).ToArgb());
                label1.ForeColor = c;
            }
        }
    }
}

}

这条线使用约10 ms。

int retval = BitBlt(hDC, 0, 0, 1, 1, hSrcDC, location.X, location.Y, (int)CopyPixelOperation.SourceCopy);