Simply click the sample button and a beautiful palette will be created based on the active comp. Colors samples pixels from all around the image, and sums them down to a compact, easy-to-use palette.

Advanced sampling options

To sample the image with greater precision, right-click the sample button. A new dialog will pop up:

From here, you can adjust the settings that determine how Colors builds a palette from your image, such as the number of data points and a threshold that filters out colors that are too similar.