Algorithm to repaint image foreground and keep shades

Question

I'm looking for algorithm that is able to do something like this:

The algorithm should change the color of an image with a transparent background. But it should not only set the color like bitmap.SetPixel(x, y, newColor). instead it should respect the shadows of the foreground.

Currently I use the following algorithm:

public static Image Foreground(this Image xImage, Color xColor)
{
    //FOREGROUND
    if (xImage == null) return null;
    Bitmap bitmap = new Bitmap(xImage);

    //LOOP PIXELS
    for (int x = 0; x < bitmap.Width; x++)
        for (int y = 0; y < bitmap.Height; y++)
            if (bitmap.GetPixel(x, y).A != 0)
            {
                Color color = bitmap.GetPixel(x, y);
                int a = color.A;
                int r = Math.Abs(xColor.R - color.R);
                int g = Math.Abs(xColor.G - color.G);
                int b = Math.Abs(xColor.B - color.B);
                bitmap.SetPixel(x, y, Color.FromArgb(a, r, g, b));
            }

    return bitmap;
}

It works well if you repaint from black to white. But if you repaint from white to orange it dosn't work well.

Is there any better solution?