The normal way to darken bamboo, called browntone, is to "cook it."  Bamboo is normally heated to straighten and strengthen the material and by using a bit more heat for a little longer than normal a brown color can be achieved.  Flaming, a process of "scorching" the outer layer of the bamboo can give an appealing appearance to rods. Unlike browntown, the object is to unevenly color the bamboo. 

In his book, Bamboo: Fact, Fiction and Flyrods, Bob Milward discusses the damage excessive heat can impart to bamboo.  Up to a point, heat will strengthen bamboo.  However, at around 400 deg. bamboo becomes charcoal.  Milward puts forward the idea that this distructive process starts with the first color change in the bamboo.  As the color changes the bamboo becomes progressively brittle until it has not strength at all. 

I've never indepently tested this theory, but my own feeings are that he's on to something, so my browntowning comes from a bottle, not a flame.   If  you'd like a darker rod, I can dye the cane to almost any color.