The reason why you cannot make a pane smaller than certain number of pixels, is the default value of the MinWidth property (20). That property sets the minimum number of pixels that the pane accepts as correct for its Width property. Even though you can change this setting by specifying a smaller value for this property, in this case, I would recommend using the slider control instead of the splitter.
As you mention that you decided to use the splitter because you need to use different colors for the three sections, I creates a simple page that demonstrates how you can do this with the slider control.
All the best,
the Telerik team