Colored shadows cast by translucent objects

How would I go about doing something like this?


I’ve followed a few guides that haven’t helped. My scene has some colored, ray-traced translucent rocks sitting on a white piece of paper. The paper is set to receive transparent shadows, but no matter what I do I end up with gray shadows. I haven’t had much luck finding answers that work for me on Google so thought I’d try here.

Blend file attached. This is just a sandbox for playing with light, etc.

Attachments

gems.blend (615 KB)