It's not the typical use of the word green when describing compost. What I mean is the use of living green plants to help prevent moisture from escaping the soil. You might be more familiar with the term weeds but in the end there will be no weeds since the meaning of a weed is a plant that is growing somewhere you don't want it to. Sure you don't want all plants to grow in the beds such as grasses but after these are removed the ramaining plants can actually help your crop by promoting competitive growth even though they aren't really competing for root space.
The key to successfully green mulch is to be selective as to which plants you will let grow. In the end what you have effectively done is companion planting. Most of us know some of the basic companion plantings but for some reason can't wrap our brains around maybe a less noticable benefit to letting some plants grow in the area.
Plants are far more sensitive to infrared light than our eyes are and since over half of the radiation from the sun hits our planet as infrared light it is important to know what our plants think about this. Infrared light stimulates growth which is why plants grow faster as the days get longer. Plants also reflect a bit of the infrared light and thus when smaller plants reflect this light up to our crop plants, the crop plants get more infrared light and, although they might not realize they are competing with space, they respond to the added infrared light and grow even faster. They push their roots down and produce more leaves. Again, they don't know that they are competing with another plant or what kind of plant they are trying to compete with. So, if you are letting plants like clover grow under the crop what you have is a competitive response to a plant that isn't trying to compete and is instead merely helping to hold moisture in the ground giving an added bonus to the growth effect.