It is possible to be achieved with a custom skin, where the background image for the sliding zone, will consist your desired colors. Tutorial: Creating ColorPicker CustomSkin
You could aslo find useful the Telerik Visual Style Builder
which allow you to create custom skin really fast. Meet the Visual Style Builder
will give the steps to use it, there is also a video tutorial to make things more clear.
In RadColorPicker demo page
there is a similar slider.
Attached is CP_slider.jpg showing similar slider.
All the best,
the Telerik team
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