It seems I need some help.
I thought I finally understand most of the functions etc in terragen now, but then I always encounter some kind of problem with the simplest things.
Right now- fake stone colours. 
For some reason the fake stones are not using the colour function colours , but the base terrain colour. 
No idea why. 
Attached the tgd file, a render and a  screenshot of the shader tree  
And some terragen 4 render tests 
A surface layer with a fake stone displacement as a child layer - and the colour function is a couple of power fractal shaders with displacements merged in 2 merge shaders. 
I had problems with masking the fake stones properly before, but it's not even that, I only used the fake stone shader's own stone density variation option . 
BTW, T4 is amazing. The atmospheric bloom and these new clouds are incredible.