Here is the node setup to detect the white area, compute the transparency from that, and remove the white fringe, replacing it with green. (mostly)

I have used Poser Pro 2014's ability to rename nodes to mark what they're doing. I mostly kept the node type so you'd know what it is, but the comment is in parentheses.

In the upper right is the Image_Map (called Color Map). You can put anything with a green or orange or yellow leaf on a white background and this is going to work.

The comp node extracts the blue channel from the color map. I'm taking advantage of the subject matter here - leaves have very little blue in them. So anything with a lot of blue is not the leaf.

The Math (offset) is offseting the small amount of blue (usually less than .2) found in the leaf itself.

The Clamp (detect white) node is doing several things. 1) It throws away the offset negative data - that becomes 0. 2) It is amplifying the fringe area where white transitions to green. 3) It is restoring the white level that was offset in the previous step to full white.

Next to it, the Math (opacity) node is inverting the white detector to make my transparency map. (As I said before, Poser is actually expecting an opacity map in its "transparency" input, so that's what I called it.)

The Blender (remove white) node is removing the white and replacing it with green so that any fringing that still happens is fringed with green, not white.

 

---

Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)