Woods with same color tones or stark contrast
For my 2 cents worth I try to look at the woods and no matter what type it is there are many times different colors and shades or tones in each type of wood. Also in many woods the sapwood has a stark contrast to the heartwood. I try to keep the woods that will be placed together within the same color with any accents being in a stark contrast to those. This is why ebony, blackwood and ebonized finials always work well with woods that are lighter and Holly and maple go well with the darker woods.
I guess the whole process is determined by each turners perception of what works in their eyes, I have seen some combos that just clash terribly and others that blend the tones and grain structures perfectly. Some pieces could be enhanced with small veneers of the "Black and White" to let the eye see it as a whole and as almost 2 separte entities that complement each other. I guess that is why I have hundreds of almost finished pieces just waiting for me to find the right piece of wood for a lid, base or finial. I hate to rush my own perception of perfection!
The great thing about wood is that you can change things along the way if it does not work at the time, and all of the proto-types can be used later for a turning that is longing for that special part.
No rules but trial and error! Just enjoy and test the waters.
Jeff