This is a popular question among many first composters or organic gardeners. Regardless of the name, "Greens" and "Browns" are not differences in physical color. It is more technical than that. These terms are functions of the C:N ratios in all once living creatures, plant or animal.

Greens and browns are nicknames for different types of organic matter to use in composting recipes.

Greens are high in nitrogen or protein, thus organic nitrogen sources. These products help the composting microherd to grow, breed, and multiply fast in the piles, thus creating extreme internal temperatures in hot compost piles.

Browns are high in carbon or carbohydrates, thus organic carbon sources. These products supply the energy and food that mostly all soil organism need to survive. Carbons also help absorb the offensive odors and capture and help prevent most of the organic nitrogen in the piles from escaping by evaporation or leaching. Carbons are also essential in the faster formation of humus from the organic matter in a composting process.

A simple test to determine if your organic matter is a "green" or a "brown", is to wet it, and wait a few days. If it stinks, it is definitely a green. If not, it's a brown.

Normal compost has a C:N ratio ranging from 25:1 to 30:1. This is considered the origin or dividing line for all organic materials.

Any organic matter that a C:N ratio smaller than 30:1 is considered a GREEN.

Any organic matter that a C:N ratio larger than 30:1 is considered a BROWN.

Alfalfa Hay can be brown in color, but it is always a "GREEN", or "NITROGEN" source because it's C:N ratio is around 12:1.

All Leaves can change from green, to orange, or to brown in color, but they are usually always considered "BROWNS" or "CARBON" sources because their C:N is on the average from 40:1 to 80:1. Evergreen leaves are higher in carbons than most leaves. Deciduous leaves are best for composting. Oak leaves (that is fresh green oak leaves, not the dry oak leaves) are an exception. They can be classed as a GREEN and should be added as a nitrogen material (Fresh Green Oak Leaves can have a C:N ratio of 26:1).

All animal manures, grass clippings, and food scraps are "GREENS".

All sawdusts, bark mulches, papers, and other wood products are "BROWNS".

Most sugar products are considered "browns" because they have a C:N ratio near 50:1. However all aerobic microbes love sugar as an quick, easily digestible energy food. So by adding a little tea made from molasses, sugar, syrups, or flat soft drinks, to your compost piles, you will increase the microbial activity and internal heating of a compost pile.

Just because an organic material has a C:N as a green or brown doesn't mean it will always act like regular high nitrogen greens or high carbon browns. The real test is in the experience of composting it. C:N ratios are just good guidelines or starting points for determining starting points for various green-brown mixtures in the pile.

For example, egg shells have a C:N around 12:1-15:1, however they act more like browns, because they may takes several weeks to decompose in the pile, based on the total amount of nitrogen in the compost pile. Seaweed is considered a green (C:N = 19:1), however it is really more a carbohydrate because it's so high in carbon, compared to its other nutrients in its makeup.

Do a web site search on "Compost Ingredients" for a more exhautive list of composting materials and their average C:N ratios.

Happy Gardening!