After tinkering quite a bit with composition and lights, I imported my 3D blockout into Photoshop. I didn't use any fancy tools, rendering passes nor compositing, it was just plaing overpainting on a single layer. This didn't make things all that easy but having a base to colour pick from and the clown ID pass sped everything up quite a bit.

At this stage I started having troubles with my computer as I realised a little too late that it was showing me very bright blacks compared to my phone - as a result, the render looks quite dark from most devices (except from my monitor, apparently). 

Many correction layers later I started painting all of my characters and creatures on the scene.