Hello,
I think I could do that with Rubens but why not displaying all friction velocities in colour with another display of heights on the same page ? It would give all the information.
For what you asked for, the solution with Rubens would be :
1) A colour view of friction velocities
2) A colour view of heights less than -5 m, choosing black or white
3) A colour view of heights greater than -3 m, choosing black or white
Then you superimpose view 2 and 3 on view 1, the black or white will cover all friction velocities that are not in the (-5,-3) range of heights.
But well, we do not maintain Rubens and it is given only for Windows, so it is not a very sustainable solution...
JMH