Dr Jigoro Kano apparently began the custom of having his ‘yudansha’ wear black obi (belts). These obi weren’t the belts judoka and karateka wear today, the judogi (Judo uniform) hadn’t been invented, his students practised in kimono, with the wide obi worn with this formal dress.
Dr Jigoro Kano introduced the modern judogi and its modern obi, but he still only used white and black belt ranks.
Dr Jigoro Kano changed the system for Mu Dan Sha (Kyu grades) and divided children (under 15) and adults (15 and older). Beginners wore light blue belt, Go Kyu (5th) and Yon Kyu (4th) wore white, children wore purple and adults brown if San Kyu (3rd), Ni Kyu (2nd) or Ik Kyu (1st).
The white, yellow, orange, green, blue, brown judo kyu coloured belts were introduced by the London Budokwai and spread throughout the world.
'Purple Belts' London Budokwai 1922