Why can't I choose any colors in Prism Mac?

Problem: You choose colors in the color picker, but the color you chose doesn't actually get used by Prism. 

Possible explanation:

When you click on a color field in a Prism dialog, that field is highlighted with a rectangle, and the color picker is used to change that color. As you pick colors in the color picker, you'll see your color in the color field on the dialog. 

But what if you double-click on a color field? The second click somehow negates the first. The color picker is still present, but it not associated with any color field. You'll see that the color field is not selected with a border. 

In this state, you can pick any color you want, but it won't do any good because the color picker is not associated with any particular color field. You'll notice that the color chosen in the color picker is not shown in the selected color field, because there is no selected color field. 

Easy to fix. Click (ONCE!) on a color field to select it. Now the color picker will work.