Solid layers are perfect for backgrounds, color blocks, or as bases for effects. Think of them as flat, colored rectangles you can place anywhere in your comp.
Steps to add a new solid layer
Go to Layer → New → Solid… and the Solid Settings window pops up.

Pick a color, set the size (use “Make Comp Size” if you want it to match your composition), and name it so you’ll find it later.

Click OK and the layer appears in your timeline, ready to animate or use as an effect base.

One more thing
Use Ctrl+Y (Windows) / Command+Y (Mac) to create new layer instantly — skip the menu entirely once you know the shortcut.
Related how-tos
How to change the background color of a composition in After Effects
How to add a new text layer in After Effects
How to add a new shape layer in After Effects
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