I am looking into the same thing. It might be a couple parts. I am thinking Creating a material then Create 3 colors one true blue one true green one true red then create three parameter that control there alphas in a lerp add them together to get your color. In the widget create three sliders one that controls each alpha parameter this should give the blending options needed. If this works you could use this to set up a full range of material variables for game and character customization. This will be my first attempt. I will try to create a color picker like they use in the engine.
Hey guys at Epic maybe you could gives access to the engines color picker as a widget. I could see this being useful for a lot of Designers:))))