I've handled hundreds of tonkin bamboo sticks over the years, I don't get them in bundles but select them from a bins. The color doesn't tell you anything, as Pat said, and its difficult to tell how old the plant was at cutting, but I have noticed a couple of patterns:
-the weaker ones are few, but often thicker
-they're lighter than the rest
-they have excellent nodes, as in flush with the rest of the shaft
-the nodes don't have oval branchlet scars
I suspect they're somewhat immature but from thicker stems and if left in the ground longer would become harder and heavier.