From the colour picker, click the small up-arrow to the right of the metalic option. From that popup menu, choose the 'LS/H picker'.
From the vertical hue picker, choose your hue.
Now, when you paint you're only allowed to choose variations on the hue from the larger curved area of the colour picker. So you can choose any variation of lightness or richness of your one hue.
What can you paint?