Noah. As I understand, this color wheel only shows colors with 100% luminocity, you also need some slider control for luminocity/brightness.
Here is how to make HSV color picker:
Clicking on the bottom sprite gives you Hue number (from 0 to 360), you need to measure the relative distance from the left of the sprite.
Clicking on the top sprite gives you both Saturation and Value numbers (from 0 to 100).
In the above example I got H=157, S=86, V=70
After converting to RGB: R=25, G=179, B=120
There are two tricky things:
1. After clicking the Hue sprite you need to read clicked pixel color and update the SV sprite with the same color. SV box actually consists of 3 sprites on top of each other. Solid white sprite at the bottom, with "Set color" effect. And two gradients on top of it, you can make them in Photoshop or Gimp - one horizontal gradient from transparent to white, and a vertical gradient from transparent to black.
2. Converting HSV to RGB requires some math. You can find it in google.
Or you can simply read RGB color of the clicked pixel the same way as in NOAH's example with Drawing Canvas.