Poops can change color for different reasons. Sometimes it’s as simple as eating more hay (but that usually makes them lighter green, not darker). Also, some pellets AND hay actually have dye additives to make the hay greener when crops are more brown. Brown hay is actually not bad (unless it has black spots and is musty indicating mold), but sometimes, due to weather, etc, hay may have more brown in it. It is still high in fiber, but just lacks the extra nutrients, but rabbits don’t eat hay for nutrition (it has so little anyway), it’s for the fiber. Anyway, most people think green hay means fresh and good hay, and some companies may resort to adding color (food grade either during growth) to keep the hay aesthetically pleasing. I do not know for sure which companies actually do this — it’s kept pretty hush hush. So it is possible that the green is coming from that. Did you recently get a new bag of pellets or hay? 

Are the poops still dry and round?   If you continue to see it and/or see other changes, then it might be a good idea just to double check with your rabbit-savvy vet, if you have one.