Poser does not do multi-colored shadows.  These are different from caustics, although they might seem similar.  Other rendering apps do not bundle them together though.

Gather node almost certainly cannot duplicate this, as it works globally, but you want your transparent multi-colored surface to cast a multi-colored shadow from the direct light, which Gather doesn't actually use (it's surface-based).  Gather is pretty similar to AO in that it is applied without regard to actual lights in the scene.  Basically Poser just can't do this, although someone is welcome to show otherwise.

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