Wherever you see Custom color picker or Custom, this is what you should do. In other words, this example is setting the Frame color. But this option works the same way no matter which item you are using it with.

Step 1: Click on Custom color picker or Custom.

Step 2: Click on the circle to choose your color [orange arrow].

Step 3: When you click on the circle, the color picker will pop up. You can choose the approximate color you want by clicking on the color spectrum "rainbow" [light green box] or color samples [arrow].

Step 4: Once you have picked your general color, click on the exact shade you prefer [yellow box]. Note that your choice is shown in the Custom color picker circle [arrow].

Step 5: Click Save at the bottom of the column to save your changes.