Hello,
Colored shadow is not in 5.6. It is something we want just like you 
but not at the top of the priority list right now. Tracing translucent colored shadow is easy in general but it will expensive. So special techniques and optimization will have to be developed.
As a reminder for everyone, colored shadow in unreal is only supported via the lightmap baker and the path tracer: Using Colored Translucent Shadows in Unreal Engine | Unreal Engine 5.0 Documentation | Epic Developer Community. RT translucency only support opaque shadow from translucent IIRC.