I have done some particles with colours in POP. While I tried to make a preview with the fast mantra renderer, all the particles came out in black and while. (but they were in colour in my Houdini viewer)
Does anyone know why?

You have to apply a shader that listens to the point colour attribute (Cd); and the default shader doesn't. Try apply a VEX Layered or VEX Supermaterial and you'll see colours.