Hi, Albert. When a green leaf turns color while still on the tree, it's because the tree is degrading the pigments in the leaf and pulling the soluble components of those pigments back into the plant to store for the next year. Nitrogen-containing molecules like protein are especially valuable to the tree so they are more heavily depleted than the high-carbon structural molecules. So as leaf senescence progresses the C:N ratio increases.
About that 25:1 ratio-- did the book specify whether that was for green leaves or brown? And was it only talking about mulberry?

* Posted by phytolacca Fri, Aug 6, 10 at 11:16 ... About that 25:1 ratio-- did the book specify whether that was for green leaves or brown? ...

Good question. I found my notes and they do not say if 25:1 was green or dry leaves. The information wasn't even from a book but rather a periodical called Science Digest.

Cornell's On-hand Farm info will give you hundreds of C:N values, and a better idea of green and browns.

The leaves of trees and shrubs will have a Carbon to Nitrogen (C:N) ratio of, normally, about 40:1 ranging to 80:1 depending on age. Optimal C:N for composting is 30:1 so if that text was correct about the C:N ratio of mulberry leaves (what I find refers to an extract of them) that ratio would change, overtime, as the leaves aged and they lost nutrients.
In response to changing light conditions deciduaous tree leaves start to loose the chlorfil theat makes them green and allows them to change sunlight into nutrients that feed the plant they are on. As that chlorofil is lost we see the leaves true colors, but whether those leaves actually loose a lot of the nutrients they had while green has not been widely researched since most people still consider them waste and of little to no value.

kimmsr, leaf senescence and nutrient resorption are very well-studied phenomena. Leaf nutrients are translocated by the plant to buds for the next year.
BTW, the color of a leaf without pigment is brown. Many trees additionally produce anthocyanins, which are red, during the senescent phase of the leaf life cycle and not during the growing season, so we don't get to see the orange and yellow carotenoids, which you may be referring to as the "true" color.
Here are a few of many papers on leaf nutrient resorption. The first paper linked is a meta-analysis of 92 other studies on N and P resorption.

Well, that's frustrating. The links got stripped out of my post. I guess I'll have to use the link box and post them one at a time.

Here is a link that might be useful: Kobe et al 2005

I have to put a message here or GW rejects the post.

Here is a link that might be useful: Potter et al 1987

link 3

Here is a link that might be useful: Cardenas & Campo 2007

Proof that green does go brown is the common compost mix of grass clippings and straw. Straw is absolutely a green grass gone brown.

Lee

At Does green go brown? billhill posts "When a green colored green dries out, it is still a green despite what color it may turn. Grass clippings cut while green and allowed to dry out and turn brown color are still a green." then four hours later tapla posts "As grass clippings dry out & turn brown, much of the N is volatilized and they end up a brown." Then twenty minutes later digdirt writes "[b]ut the consensus is the reverse. Dried grass is still more of a green than it is a brown". About six minutes later dchall_san_antonio writes "dchall_san_antonio "Rather than thinking of green and brown, carbon and nitrogen, I think in terms of protein and carbohydrate." A few hours later bpgreen writes "[i]t depends on whether the leaves turned brown on the plant or off." The following day kimmsr writes something.

The person who first published this green/brown nonsense should be put in one of those large animal composting contraptions.

I agree, Albert. If a material has a relatively high protein content, then it is a "green" and will heat your compost engine. If it does not have much protein, then it is a "brown" and is very useful as a source of carbohydrate energy to the microbes in the pile. When the protein is depleted from a material, then it starts to turn more toward the "brown" state.

Lettuce, which looks green, is a "brown." Coffee grounds, which look brown, is a "green." Nuts, beans, seeds, and grains are "green." Leaves, stalks, and woody parts are "brown." Grass is different in that it stores protein in the blades until it goes to seed. Then the protein concentrates in the seed. If you mow it and collect it before it goes to seed, then you will have a lot of protein in the grass.