Hello again!
Having background colors is a nice option, saves having to make an image in the right color, exporting it, etc. How about adding gradients as well? Like the usual gradient types, same as available for shape fills (well, radial gradients would be needed as well) - since you already have the technology for shapes why not add it to backgrounds? Sure you can put a shape in and add a gradient to it, but it can't be the base layer and it's a bit awkward as well.
It could look like this:
