C#

public static Bitmap Diff(Bitmap src1, Bitmap src2, int x1, int y1, int x2, int y2, int width, int height)
{
    Bitmap diffBM = new Bitmap(width, height, PixelFormat.Format24bppRgb);

    for (int y = 0; y < height; y++)
    {
        for (int x = 0; x < width; x++)
        {
                //Get Both Colours at the pixel point
                Color col1 = src1.GetPixel(x1 + x, y1 + y);
                Color col2 = src2.GetPixel(x2 + x, y2 + y);

                //Get the difference RGB
                int r = 0, g = 0, b = 0;
                r = Math.Abs(col1.R - col2.R);
                g = Math.Abs(col1.G - col2.G);
                b = Math.Abs(col1.B - col2.B);

                //Invert the difference average
                int dif = 255 - ((r+g+b) / 3);

                //Create new grayscale rgb colour
                Color newcol = Color.FromArgb(dif, dif, dif);

                diffBM.SetPixel(x, y, newcol);

        }
    }
    return diffBM;
}