Phew, this is consuming hours on end! One more good session should get me all the way through the leaves. I think then I can go back over them all to solidify.
Stem color has been a real challenge. I found that straight Violet Hematite from Natural Pigments is a rather close start. There is a lot of stalk texture to add that I think will center the leaves.
Here's a great article on egg tempera that I recently ran across in the studioproducts.com newsletter.