This is an image I created of a cavern. the camera looks out from a passage into an open chamber where the roof has caved in. The chamber was created by using bryce rocks set negative and grouped with a positive bolean terrain to make the tunnels. There was no detail in the chamber till I used a blue light but it made the tunnel look unnaturally bright. I switched the light to negative and highlights showed up pink and the stlagtites had pink bright shadows. I guess a shadow from a negative light is bright. I turned the light red and the resulting darkness had a blue cast. I turned shadows off and got the image you see. Negative lights seem like a great solution to lighting problems I have and the render was only 45min.