The arrow stays on the center color, but you should be able to tap on the individual color squares which, as you move across them, changes the color circle at the center of the color wheel. With the exception of the middle color, the other four are all sort of "on top of each other" until you start moving them.
Give it a try and let me know if you understand what I'm saying. (Don't forget, if you don't want to save your changes, tap the X on the lower left side of the screen.)
Let me know if I've misunderstood your question.
Hold down on the color that you want to use as your Base Color. You'll get a pop up that tells you to tap the color.
Ta da! Now it's your base color.