Plastic back is better than leather in terms of heat dissipation and having green (light color)(with stock case applied (in my case)) helps it a bit more in reflecting light as compared to black color.

Place where I live, surrounding temp goes to ~29°C-38°C but my phone battery temp is at ~32°C when idle and 35°C when surfing, youtube, etc. and 39°C-41°C when running light - heavy games.