Why would you want to change the background color? Or rather... What colors would you want to change it to?
The main reason is the following feature you posted into the list:
What's not done yet in the Universe Sandbox v2 beta (build 518)?
- Adding color and texture options for bodies
I just think it would be of advantage if the user would be also allowed to change the background colour
in order to create certain color contrast if you are even planning to allow him to change the color of the objects self.
Other topic:
A) It might be good if the user would be allowed to quickly change the rotation direction of an object around an attractor in the same time while he creates it. A system providing simple controls for the user would be the following:
As soon as the user dragged the object on the position he wants it, he clicks mouse1, but holds the button. Two arrows tangentially aligned to the object's orbit show up now. The user now continues holding mouse1, however when he drags his cursor he doesn't move the object anymore, but instead just moves around the window with his cursor. What he can do now is dragging the cursor over one of the two arrows. As soon as the cursor is over one arrow the covered one will light up. If the user now releases mouse1 the body will be spawned and will automatically orbit the attractor into the direction of the arrow the user lighted up before by dragging his cursor over it.
To make the use less non-specific but more general I also suggest that if the user doesn't hold mouse1 after setting up the position of a body, he automatically spawns a body orbiting the attractor into the direction he set to a previous body. If there is no previous body the orbit will automatically assign a preset default direction, like it's now in the current state of the beta. I also do have some ideas how to realize a simple interface allowing the quick creation of orbits involving movement in the z-axis, but I might explain it later if there's any interest into it.
Anyways, I attached a sketch of the orbit direction changing function I tried to concept in this post.
B) If you are also planning on adding more complex velocity manipulation tools I also suggest you to add a button which can turn auto-orbiting on or off.
C) I also just got an interesting idea if you are thinking of a velocity vector drawing mode. The idea would be that this mode automatically could be available when auto-orbiting is turned off. When you now perform the same actions as to manipulate the orbital direction as described above you will instead stretch and rotate a velocity vector arrow above the plane. The length would indicate the speed, the direction logically the velocity direction. In order to make such a tool easy and controllable I got the following good two ideas: The colour of the velocity vector arrow could dynamically change, indicating what kind of speed your going to set your object. Green could mean that you are near the ideal orbital speed, red means that you are about to hit or you are above the orbital escape velocity. Besides that the length of a velocity vector could also be displayed as a dynamic counter besides it's point of origin.
The following picture displays idea A) and B):