Steve, that's interesting. Same light green color.
I'm now almost certain it is nutrient related issue. Since I started spraying them with water soluble fertilizer that should have all major nutrients and whole bunch of micro elements, needed by plants to grow, some of them started to darken a bit.






One of the seedlings was getting totally yellow at one point and like I said, I've started spraying it with fertilizer. Since foliar feeding enables it to receive nutrients even if soil is too alkaline or acidic, there should be visible improvement. Now it has darker leaves (oldest leaves are still the same color, fresh foliage is looking greener every day), but leaves are still a bit damaged before they unfold. They seem to dry out on leaf edges (second picture from below).
I'll see if darkening continues and if I can cure leaf damage on one of the plants.
One of pale green bamboos is now starting to shoot. There are four shoots emerging at this time - so far it looks like they will be smaller than dark green seedlings shoots (4 shoots, just like this one). Slower upsize, with much more troubles growing. I've had several seedlings that almost died (some are still struggling) because they were not growing properly.
I used the same soil in all containers. If there is nutrition problem, it might be caused by bamboo deficiency, perhaps in the roots. One of the seedlings (last photo) managed to grow normally green shoot. Looks like they can be cured by themselves, but most of them don't even survive the first few shooting cycles.
Seedlings from same seed supplier on one of french bamboo forums showed same problems. Many bamboos were sickly pale green or yellow.