Amtorian jiu-jitsu was a martial art known to the Tenth Doctor. It was considered to be a dangerous practise, as even watching it could cause headaches, nosebleeds and much worse.
Like Earth martial arts, skill level was denoted by coloured sashes. The highest level was denoted by a purple and puce coloured belt. In order to achieve this rank, the trainee would be attacked at any time to see if he could defend himself. Though the Doctor was good at Amtorian jiu-jitsu, he never took the challenge for the final rank. (PROSE: The Story of Martha: Star-Crossed)