Normal Judo is called "Kodokan Judo" in Japan.
Before WW2 it was called Kodokan Judo of Standing, Kosen Judo of Ground.
They were like the competition and sometimes fought.
But after WW2, GHQ did not permit people to do Budo ( Martial Arts like Judo and Karate and Kendo and so on ) in Japan.
Kodokan Judo went to foreign countries to spread Judo.
So nowadays Kodokan Judo is main in the worldwide.
But Judo became sports that uses point system and weight distinction system.
Judo is going to different direction.
Kodokan objected colored jacket.
But colored jacket was decided.
Originally under Budo ( Martial Arts ), there is no distinction between fighter's weight.
Small body person can win big body person.
That's Budo.

Kosen Judo started to be known lately, after Gracie Jujitsu or Yuki Nakai appeared in Japan.
Yuki Nakai learned Kosen Judo when he was a National University student.
Kosen meant range of old high school to junior college or college.
After WW2 Japanese school system was changed by GHQ policy.
So there is no Kosen anymore.
Now it became university.
So it is called that some National Universities are took over Kosen Judo tradition.
Kosen Judo was around 1914 to 1943.
Ko of Kosen means high.
Koko means high school.
There was no coach or master to teach Kosen Judo at all in that time.
So students created ground technique by themselves.
It is called that can be strong at ground, if body is small and practice more one hour than others.
They practiced to win other schools.
The regulation of rule is few.
So it can adopt Vale Tudo fighting style.
Under normal Judo, if stuck few seconds at ground, judge says "mate" ( wait ) soon and have to start from standing again.
But Kosen Judo, if opponent is in turtle postion ( quater postion? : I don't know how to say in English ), can keep fighting.
Refer to left picture of below.

Master Kanae Hirata said below.
"Students practiced pretty hard in that time.
During pracitice, after their body got dull, real practice started.
Almost Kosen students had been studying only study until enter Kosen.
So almost were small body.
Under ground, small body people can win, if they make effort.
Under ground, It's legs' power than arms' power, It's brain's power than legs' power.
That's Kosen Judo."