Why is the green river Wyoming formation green?

It is not green. It is named for the location it was first described, the Green River which is a river in Colorado, Utah and Wyoming. The Green River is 730 mi long and the basin is 48,100 sq mi. The formation is made of lake deposits that are from the Eocene, 53.5- 48.5 million years ago. It is famous for well preserved fish fossils and oil shale