It is still based on a standard Mandelbulb.
But the geometry is turned "inside out" as I call it.
Rainbow effect is a direct presentation of the ray marching cycles count colored by the position in RGB space.
I am also using this count to create false shadows and atmosphere like enviroment like in this next video
https://www.youtube.com/v/QAE9T7IgMfo&rel=1&fs=1&hd=1