This may not be a bug (maybe just the way it works) but if I add a container then try to set a background colour it doesn’t display the colour until I drag something into the container. You can see in the screen shot the container should have a red background but it doesn’t display. As soon as I add an element it shows. Not major but I’d consider it unexpected behaviour.
This is a tricky one 
You’re not seeing the red background as the drop zone is taking up the entire Container due to there being no padding. Were you to add some padding you would see the red background.
The drop zones need to take up the entire droppable area of a component so you know where you can drop a component in to.
I can see why this might be confusing though. We’ll see if we can improve on this in a future beta.