In my experience with a few wild-caught rough greens, this has often leaned towards a beautiful turquoise blue showing up in the skin between the scales on the snake's body. As was noted earlier, green snakes are somewhat arboreal in nature, the rough more so than smooth, so their caging requirements must reflect on this. These tanks are often tall enough to provide the arboreal setting of overhanging branches that is needed. One of the tricks to keeping green snakes is making sure you provide them with plenty of the hiding places that they require due to their shy nature. Normally, hiding areas for captive snakes would simply be provided by a hide box, but for green snakes that is not the best option.
Green snakes do best when they are provided with lots of leafy greenery to blend in with.
This can be accomplished with making use of houseplants, such as live philodendrons, that willingly vine all over the place, or you can easily use fake silk or plastic plants from a craft store. Fake plants are much easier to care for and clean than are live plants. Calci-sand works well as a bottom substrate and is heavy enough to help anchor some fake plants in place. Being diurnal snakes, some UV fluorescent lighting is necessary for their well-being. Never set a glass tank in front of a window where the sun can bake an animal to death in a short time.