The easterns I've seen from around Oakland and macomb counties come in all shades of tan to bright yellow, green to dark brown to nearly black. 'Christmas' is the clearest green I've ever found. Shades of olive and brown with varying degrees of yellow are more common mid range colors. Most have a bit of red along the neck but it is uncommon for it to go beyond the first third of the body length, or to be particularly intense.

Im not sure polymorphism is the right term as they tend to be variations on the same color scheme- the intensities saturation and proportions seem to vary along pretty natural curves by locale.