Hi Rayhane,
At present, we do not have the support to change the background color of the Image obtained from the SfImageEditor control. We have already added the feature request for erasing the free hand drawing support in the image editor along with that we have considered your feature request to modify the background of an image in image editor .You can track the status of the feature from the below link.
Please cast your vote to make it count. We will prioritize the features every release based on the demands and we do not have an immediate plan to implement this feature since we committed with already planned work. So, this feature will be available any of our upcoming release.
If you have any more specification/suggestions to the feature request, you can add it as a comment in the portal.
Regards,
Ashok Kumar Viswanathan.