Japan's Autumn color is wonderful. Especially " Karasawa" is very very famous in Japan.
You can enjoy a lots of color at the same time. Red, Yellow, Green, Blue ( Sky ) !
There too many people at Karasawa during this period. Normally approx 1st & 2nd week of Oct. If you wish to stay mountain hut at Karasawa, you only have tiny space for sleep. So I will not recommend to stay mountain hut at Karasawa at begging of October. You may be shocked to see this situation.
So if you wish to see this autumn color, I recommend to stay at Yokoo ( required booking 1 month before )or Tokusawa ( required booking at earlier stage.)