Why does one judo player wear white and the other blue?

• Colours: each athlete brings a white and a blue judogi. Reversible suits are not allowed.
• How it is worn: the jacket crossed left side over right, the belt tied tightly above the hip bones.
• Under the jacket: women must wear a plain white short-sleeved T-shirt with a round neck. Men may.
• Judogi control before every contest checks, among other things:
  • that the suit is dry, clean and not worn out, especially at the collar and lapel;
  • the IJF approval label on jacket, trousers and belt;
  • that the sleeves cover the wrist bone, and that there is enough room between arm and sleeve;
  • that the jacket covers the buttocks, and the length of the trousers;
  • that any knee or elbow supports are soft, with nothing rigid or metallic.
• Failing it: the athlete has one more chance. If the suit still fails, the athlete fights in a reserve judogi supplied at the venue and may not have a coach in the chair for that contest.
• The body: long hair tied back so it does not get in the way of a grip, nails cut short and no jewellery, piercings or head coverings.
• A hard or metallic object worn in a contest is hansoku-make.
• The judogi must stay on, properly worn, until the athlete has left the field of play.

A tight jacket or short sleeves give the opponent less to grip, which is an advantage in a sport built on gripping the jacket. Measuring every suit keeps that from being decided by tailoring. The two colours are for the referee, the scoreboard and the spectators.