This script will setup a script controller on the scale controller of the bone. The script controller will monitor whether the bone is being stretch below or beyond a set value, and color the bone to a user defined color.
The concept is taken from the free tutorial by Chris Thomas at; http://www.cg-academy.net/pages/free_tutorials/tut_scripted_color_to_length/tut_scripted_color.php
Just drag and drop into 3ds max. The script can accessed be under the "Toke Scripts" category.