How to manipulate images at pixel level in 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;

}