A solid fill turns outlines into bold, clean shapes that stand out on your canvas.
Steps to fill shapes with color
Select your shape layer. In the toolbar, choose Fill: Solid Color and pick the color you want.

If you already have the shape drawn, open Contents and change the color there instead.

One more thing
Switch the type to Gradient for smooth color transitions without adding an effect. It gives you more control and keeps your layer cleaner.
Related how-tos
How to create shape layers in After Effects
How to create a rectangle mask in After Effects
How to add strokes to a shape in After Effects
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