A simple 100mm ruler.
For people that do not own Bambulab printer with AMS system, you can simply add pause at layer #9 to swap out the filament to a different color.
You can dowload 200mm ruler from here: https://makerworld.com/en/models/386026#profileId-286387
You can download triangle ruler from here: https://makerworld.com/en/models/388125#profileId-288847