The Color Picker field picks any color in a color picker.
To restrict the choice to the colors of your theme, use the Color Palette field.

Configuration
Go to Modern Fields > Fields groups and add the Color Picker field. Its settings are in the inspector:
| Effect | |
|---|---|
| Enable opacity | Lets the user set the transparency of the color. |
| Show clear button | Adds a button to empty the field. On by default. |
Display the color
The field stores the color as a CSS value, such as #e5554e. Read it with get_field():