Right checked you wire colours. All matched up to mine just different labels.
At the ignition switch:
Red - Aux. you can use this at a later date to connect your radio switched live. ignore for now.
In1 (Brown/White) & In2 (Brown/Purple). On mine these was BatA & BatB. Twiddle these two together and connect into connection block with the Marlin Brown. This is temporary for testing.
IGN 2B (Green) ignore.
1 (Black/Yellow) & IGN2A (Yellow). I only had two Marlin greens. So connected both to the 1 (black/Yellow) wire. But as you have three, Connet any combination of the greens to the black/yellow and Yellow. For example Two greens on Black/yellow and one on Yellow. As long as the greens are connect to either of these wires.
White/Red - to Marlin white to Starter Motor.
At the business end (the engine bay
Connect the Brown to the battery. I went via the Rover engine bay fuse box but in the rear of the Marlin. If that makes sense. 40 A fuse.
Can connect the Purple Wire to another fuse on the fuse box. But does not need to be done at this stage.
Bolt Marlin blacks to chassis.
Connect Marlin white to Starter solenoid. On the Rover this went via a relay. Not sure on the Honda.
Connect big red lead from starter to Battery. On the Rover there is a big black lead from gearbox. This goes to the negative of the battery.
At this stage, you should be able to crank the engine.
On the Rover, the green connected to ignition sense on the ECU.
I have emailed you the diagram on the other connections using the Rover ECU. Can't comment on the Honda ECU as I've not seen it!