I don't think there is a clear cut definition of "green" chemicals. Right now it appears it is whatever the chemical manufacturer wants it to be. In a sense it is more a
marketing ploy than anything else since it is the "hot' button companies use in many different industries.
Almost all chemicals used in the car wash industry could be classified as biodegradable since the basic definition of that is that the ingredients will eventually degrade and return to a natural form. The other term often thrown around is "environmentally friendly" which is usually used in the same sense as biodegradable.
To answer your question I guess you need to check out what is available to you in your market and go with what you think will appeal to your customer base.