Create a User Color node
Select User Defined... from the Diffuse color pop up menu (the first Diffuse menu).

Change the color space
The User Color dialog will appear. It has three inputs - one for each component making up the color space you currently have selected. All three of the nodes are value nodes. These values are combined to make a color. The default color space is rgb. Change the color space from rgb to hsv by selecting hsv from the color space pop up menu located in the upper left corner of the dialog.