A gradient ramp will give your background color a pro look — it’s way more interesting than a flat solid.
Steps to add gradient ramp background
Create a solid layer by pressing Ctrl+Y / Command+Y.

Apply Effect → Generate → Gradient Ramp to it.

Pick Start and End colors, then adjust ramp direction with the points to control where the colors flow.

One more thing
Use 4-Color Gradient for more complex blends with multiple color stops instead of just two.
Related how-tos
How to create a blur effect in After Effects
How to add adjustment layers in After Effects
How to apply an effect in After Effects
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