Basic color controls in Final Cut Pro

Before you expand the color controls, you see the following:

Color well: A small color swatch that, when clicked, opens the macOS Colors window. See Pick a color in the Colors window. You can also click the downward arrow or Control-click the color well to open the pop-up color palette. See Use the pop-up color palette in Final Cut Pro.
Disclosure triangle: A control that, when clicked, reveals individual Red, Green, Blue, and, when applicable, Opacity sliders. See Use expanded color controls in Final Cut Pro.

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Final Cut Pro User Guide