There's a lot of discussion on the internet about how to tell the quality of soap nuts and some sites claiming one color or shade is better than the other for cleaning. The color of soap nuts, while one of the factors in determining quality, is not an indicator of how much soap is present. Pure saponin (the surfactant or "soap" that does the cleaning) is light yellow in color, not a dark brown as improperly reported on other sites, thus brown soap nuts do not contain more saponin than lighter colors. The color also varies by region where it was grown (and also by species and variety) but it can be a loose indicator of age and/or if the soap nuts were properly stored. The information on this page refers to the Sapindus mukorossi species.
All soap nuts start out green on the tree in fall, then turn yellow or bright orange in winter when they are harvested. The soap nuts slowly turn a dark reddish brown to dark brown over time and will eventually turn black if not stored or packaged properly. Once again, there are variations in the colors that can differ by region so this is approximate. While very old black soap nuts still contain saponin, they may not hold up for more than a few washes. In this photo, the 5 year old black soap nuts were packaged and stored improperly.
To properly store soap nuts, keep them in a cool dry place away from moisture. For long term storage, keep them in an airtight or vacuum sealed container.