It could also be the natural sugars & starches in the bamboo caramalising from the heat of the laser.
We have bamboo flooring in our house and the company makes 4 different shades, from a natural blonde colour to a dark chestnut. They achieve this by effectively cooking or steaming the boards for different lengths of time to caramelise them. So there are no stains or dyes used, the colour is consistent all the way through & they can be sanded & refinished if needed. Lovely product.
www.bamboozle.com.au