Hi Slobodan,
Currently we do not have support to provide Backcolor of the QRBarcode in Xamarin.Forms, we will consider this usability feature request. We will implement this feature and it will be included in our Volume 2, SP 1 release which is expected to be available at the end of May, 2017. We request you to visit our website periodically for feature related updates.
Please let us know if you need further assistance on this.
With Regards,
Gayathri R