This is an absolutely fantastic effect, and something I have been looking forward to for the past several engine versions. Using your description, I’ve managed to enable it in the lumen scene, but not in the primary view (with RT shadows enabled, megalights disabled, and otherwise normal behavior).
Colored shadows are visible in the lumen scene and casting GI as expected, but not in the primary view. r.RayTracing.Shadows.Translucency produces the expected greyscale shadows, is there a means to force them to color as well? There is additional artifacting present in the GBuffer RT shadow that’s not in the lumen scene shadow.