http://pixologic.com/zclassroom/homeroom/lesson/military-assets-with-joseph-drust/ (first part of the lesson)
I did same thing as the author of that zbrush lesson, merged subtool with one being white “dynamesh subtractive” polygroup, then unchecked all the dynamesh buttons like “polish”, “project” etc to be exact same as on the movie, then used dynamesh and the white polygroup just changed colour…
What am i doing wrong?
Does your white polygroup have more polygons than the other? If yes, try to dynamesh it to a lower polycount before using as Subtractive
After first unsuccesful subtractive dynamesh it changed to normall colored polygroup and i believe dynamesh made it with same resolution as the other (main) polygroup. Then i selected it as a single visible polygroup and turned it into white sub polygroup again, and tryied dynamesh, still to no avail 
So that cant be the problem 
Ow, did a new test, and this time with new objects it worked. Perhaps it is indeed related with polycount. This time i subtracted polygroup that had significantly lower polycount than the main one.
Thanks!