From distant memory, the dark green and red is the starter solenoid switched earth controlled by the clutch lever switch. The solenoid is also switched by the neutral switch via a light green and red wire, so it will crank as long as it's in neutral, but if in gear, it shouldn't crank unless clutch lever is pulled.
Bit strange if it's connected to a brown wire though - that's usually a switched live !
Sounds like you have some interesting wiring to check