Board Thread:General Discussion/@comment-6302057-20160307065615/@comment-6302057-20160314112402

Based on the promo for "Devil's Due", I believe that the green river is the Styx. In the Disney film Hercules, Meg's soul ends up in the Pit of Death (formed from the Styx). In the promo, Hook is being lowered into a body of water that appears to resemble the Pit of Death. Hence the green river is likely the Styx. Now that's two rivers that we have identified by colour in the Underworld.

And CoolDudeAl, I think you're right. I think that the "worse place" is Tartarus, and Olympus is basically taking the place of the Elysian Fields.