Why are my plants new leaves light green?
This is a question that comes up quite frequently, especially with smaller plants. It is not uncommon for a grower to provide more nutrients than a plant really needs, especially with smaller plants.
If your cannabis plant is growing new leaves, that show no other sign of damage or other irregularities, and the new leaves are coming in a brighter green than the rest of the plant the most likely cause is that you have provided more nitrogen than the small plant needs. As long as the leaves are not burning near the tips this is not necessarily a bad thing, although i would advise that you reduce the nitrogen for a little while.
You can see in the picture some of these plants show lighter center leaves, these plants show a very mild form of excess nitrogen uptake.
Again, not something I would worry about. Flush the plant with clean fresh water and reduce the amount of nitrogen until the plant reaches a larger size.