Bear in mind that some materials will not darken as much as others. Cork, for instance turns very dark at much lower power levels than bamboo or oak. Maple will turn a golden brown, but other woods will darken much more. There is a post here which addresses this slightly. Read down to where I ask about why they used the process they did.
Bamboo is quite hard, and may not darken as much as you want it to without some kind of treatment.