Currently it is not possible to set an alpha value for a color in the RadColorPicker. It could be useful to add such a feature in the control.
This will allow the developer to disable postbacks, set the color with JavaScript on the client alone (or let the end user set it in preview mode), while postbacks are generally enabled (i.e. the server ColorChanged event will be fired only when the Apply button from the RGB palette is clicked or after the developer restores the AutoPostBack to true).