Ah - great to see you feeding the delightful Maple and Dennis some greens. I am a well documented fan of the greens - all my buns have been fed greens and they all love them. In fact, Lillian is chomping her way through a leaf as I type.
I have found from my own experience, that they do prefer the dark green leaves best. They probably have the most flavour and if I put lighter coloured leaves in, they get left.
So, what I do for Hillian, is this - I get 3 or 4 leaves at night time, rip them up to reasonable sizes, and get a small handful of fresh parley and put that in the middle of the leaves. Then roll the leaves up making a sort of cabbage/parsley roulade, then cut it into portions and put it out for Hillian. It makes them work a bit for what they want and they absolutely love it.
I used to feed Kale - it is full of vitamins but it also gave lillian a runny tummy. Spinach too, is good but again, only in very very small quantities so they get a small few leaves of spinnach every few days, no kale, greens and parley twice a day and the occasional carrot shaving.