You could cut the top off about 2" or 3" down and biscuit joint it
with a darker piece of 2" in between the top and lower body.
This will give a nice band going around the top.

Maybe I am not understanding you. If I put horizontal grain walnut in
between two pieces of vertical grain butternut it should crack when the wood
moves.
But maybe you meant something else...

So why can't you put a stripe of vertical-grain walnut in the
vertical-grain section, and horizontal grain for the back? It might
take some careful planning to cut the top off your finished piece and
glue up a chunk of walnut, but if you glued up a piece that was 6" high
x (depth of workpiece) long, you could then cut that down to two 3"
high pieces. I think a 2-3" stripe of walnut could look really nice,
especially if the base were walnut also. I love the contrast of
butternut and walnut - obviously different colors, but very similar
grain patterns and there's a similar "warmth" or "value" or "tone" or
something between the 2 colors. (As you can see, I don't know the real
words to describe the various aspects of colors, but do you know what I
mean?)
Good luck, and let us know what works,
Andy