Hi @seank1968 I’ve been testing this and I don’t think it’s possible - you can make a specific colour transparent, but there’s no way to select a background colour. I’ll move this over to the feature request section of the forums for you Don’t forget to upvote your thread!
In case anyone else wants to do this here’s how I eventually did it:
1 - Get the image
2 - Edit Image - Get Information
3 - Edit Image - Create Image - use the width and height from the previous node and set the background color (in this case #ff0000
4 - Composite - use the created iamge as the lower image and place the original on top! Set the file name and format.