Bring back color in plots?

Previous versions of **FFTrees** used color to _distinguish between cues_ in plots.  While it makes sense that plots also work in black-and-white (e.g., for scientific publications) and the subtle mix of color and transparency in the present plots is neat, here is a question to users: 

- Does anyone miss an option for setting colors when plotting (e.g., with `showcues()` or `plot()`)?  
