Can I change the color of the background behind cameras?

If I have several cameras that draw to non-overlapping rects and theirs space between the rects that currently renders as black, can I change the black to some other color.

The only way I can see to do this is to add an extra camera which spans the entire player screen area (this is the default for new cameras), set its "depth" to a value less than all your other cameras, and set its "Culling Mask" to Nothing. You can then adjust this camera's background colour setting.

You should also remove the "Flare Layer", "GUILayer" and "Audio Listener" components that come attached to the camera GameObject by default.

Since the camera is rendering nothing but its background colour, it shouldn't incur much of a performance hit at all.

Answer:

Way1:
camera → inspector element → background color

Click the camera then inspector elements will be open then change the camera background color, its will be change.
Way2:

function OnGUI()
{

GUI.backgroundColor = Color.yellow;

GUI.Button(Rect(10,10,70,30), “A button”);

}

It doesn't necessarily render as black. Unless you use a camera in the way Duck mentioned, it's possible that you'll see whatever is cached in the GPU. This may have been fixed in Unity recently, but this was the case at least up to version 2.5, and I haven't seen such a fix advertised as a feature. I'm pretty sure I've always seen black in the Unity and Unity iPhone Editors, but I've seen the GPU garbage in webplayers and standalones, and definitely, on the iPhone, this will happen every time, with the current version of Unity iPhone.

Long story shot, always use a background camera.