NGUI widgets color is effectively baked as vertex colors when the widget geometry gets created and sent to the GPU. It's not a part of the material.
Furthermore, since you didn't specify an explicit material on your sprite, it means NGUI will batch this sprite with all the other sprites using the same atlas underneath its parent panel. So there is no "one" draw call for the sprite. There is one draw call for all the sprites -- and as such only one material. You can listen to UIWidget.onRender to get the final material, but like I said it will be a batched material for all of your widgets in the batched draw call.