By default, your composition background is black, but you can change it easily for better visibility while you’re working and animating.
Steps to change the background color of a composition
Go to the top menu and click Composition → Composition Settings (or press Ctrl+K / Command+K).

In the Basic tab, click the Background Color box and pick any color you want.

Hit OK and the background updates instantly. You see the change right away on your canvas.

One more thing
If you need a background color that’s guaranteed to stay consistent and easy to adjust later, add a Solid Layer instead of relying only on the comp background.
Related how-tos
How to set comp resolution, fps, and duration in After Effects
How to add a new solid layer in After Effects
How to add a new text layer in After Effects
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