There are many kinds of bamboo, and often the culm colours change as they mature. Not always darker. There are quite a few that emerge green and change to yellow. I have one that emerges with a bluish bloom, reveals green under that when it washes off, then ages to a dull yellowy-buff, and eventually, especially if in the sun, can turn reddish-brown. I have a yellow one that obtains red patches in cold weather, that fade when it warms up. I have a yellow one with green stripes, and the green stripes disappear in the winter and come back in the spring. It is easy to get obsessed over all these colour patterns and changes...