Change color of bitmap

BitmapEx.cs

public static class BitmapExt
{
    public static void ChangeColour(this Bitmap bmp, byte inColourR, byte inColourG, byte inColourB, byte outColourR, byte outColourG, byte outColourB)
    {
        // Specify a pixel format.
        PixelFormat pxf = PixelFormat.Format24bppRgb;

        // Lock the bitmap's bits.
        Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
        BitmapData bmpData =
        bmp.LockBits(rect, ImageLockMode.ReadWrite,
                     pxf);

        // Get the address of the first line.
        IntPtr ptr = bmpData.Scan0;

        // Declare an array to hold the bytes of the bitmap. 
        // int numBytes = bmp.Width * bmp.Height * 3; 
        int numBytes = bmpData.Stride * bmp.Height;
        byte[] rgbValues = new byte[numBytes];

        // Copy the RGB values into the array.
        Marshal.Copy(ptr, rgbValues, 0, numBytes);

        // Manipulate the bitmap
        for (int counter = 0; counter < rgbValues.Length; counter += 3)
        {
            if (rgbValues[counter] == inColourR &&
                rgbValues[counter + 1] == inColourG &&
                rgbValues[counter + 2] == inColourB)

             {
                rgbValues[counter] = outColourR;
                rgbValues[counter + 1] = outColourG;
                rgbValues[counter + 2] = outColourB;
             }
        }

        // Copy the RGB values back to the bitmap
        Marshal.Copy(rgbValues, 0, ptr, numBytes);

        // Unlock the bits.
        bmp.UnlockBits(bmpData);
    }
}