Quote - IIRC, there is a way to make Poser generate coloured shadows, with shader trickery. Unfortunately, I can't remember what it is. :(

Again, you can do it with any form of ambient lighting or shading.  I mean, define what a shadow is, and think about how rendering works.  A shadow is the absence of a light, not something positive.  It's not an addition to shading.  Shadows don't make an area darker or color it.  Shadows are simply areas where a directional light isn't illuminating.  What it can't reach. 

Rendering works by adding up different effects.  Ambient color, diffuse shading, sss shading, specular, reflection, refraction, etc.  Again, shadows aren't "black" by default.  It's just that you start with nothing, and then add effects from there.  If you want a color where directional light isn't, then you color whatever is adding to shading where directional light isn't.  Ambient color.  IBL. Whatever you want.   IBL is just easier than ambient color because it works on the whole scene, so you don't have to start changing the ambient color of every material involved. 

The one thing I know of that's actually light dependent is when you start playing with negative intensities and color values.