Additional color chooser systems are available in the Color > Modifiers sub-palette. All four pickers may be open and active at the same time.
Drag within a Color Selector to pick a color. You can also click within the Color Selector and drag to any point on the canvas or interface, and select the color at that point.
By default, colors picked from the canvas are the pure colors unaffected by lights and materials. To pick shaded/material colors, hold the Alt key while dragging.
Additionally, you can press the C key at any time, and ZBrush will pick the canvas or interface color underneath the cursor.