Houdini Quick Tip: PART 2 Controlling Light Per Component and Indirect Contribution Per Object 

That is highly dependent on your lighting set up. The color and intensity of shadows is dictated by the color and intensity of the other lights in your scene. In a pbr renderer like mantra you don’t really control your shadows separately. There is no trick to use “fake caustic” on a mantra material. This will occlude less light, allowing for lights to pass through your material. My last suggestion is to use compositing to achieve this. I always render lighting passes and rebuild from my light aovs. I believe in mantra (I have moved on to karma since 18.5) you can output shadow maps per light. This can also be used in comp to make you desired affect. I hope this helps.