Paint.NET :: How To Match Colours

When something with transparency covers part of an image (Mandatory process)(Such as the transparent layer in Twitter that distort the background), if I want to retain the original image look, I have to change part of the image into a more virbrant colours as before it was covered with a mandatory transparent layer.
 
Here is a demonstration In this case, I want to change the upper part so it looks the same colour as before, so I will need to change the upper part with more contrast or brightness, etc, so after the layer is added, it looks the same as if without the layer on. A way to do this as I mentioned, is to use curve tool or change contrast and brightness, but there is no way that I can get it to 100 accurate.