In Motion, do one of the following:
Choose Edit > Project Properties (or press Command-J).
Select the Project object at the top of the Layers list, then open the Inspector and click Properties.
The Properties Inspector opens. Two parameters in a project’s Properties Inspector affect the background color of a project and affect how a composition appears when exported out of Motion.
Set your project background properties using the following controls:
Background Color: Use this color well to set the color appearing in the canvas when no other object obscures the background.
Note: When you export a project with a premultiplied alpha channel, Motion always renders against black.
Background: Use this pop-up menu to set whether the background color is rendered as part of the alpha channel. If this menu is set to Solid, the background color creates a solid alpha channel. If it’s set to Transparent, the background color does not render as part of the alpha channel. In either case, the background color is visible in the canvas.
For more information about transparency, see Alpha channels in Motion.