Well, you can now shade any volume grid using a ramp, but the main problem is that the velocity is a vector grid with three components, and the density is a scalar grid. Attached you can see how I can visualize the vel.y component using false colors, but the problem is that this is not the full velocity magnitude ... if you need this feature, I could add an option in the plugin to compute the velocity magnitude from the three velocity grids vector, to use them in the scalar grids.
Octane currently doesn't support custom geometry attributes. This is planned feature along the 3.x dev cycle, but I will try to find some workarounds if this feature is delayed.
Computer Graphics Software Developer
Octane plugin development and support: LightWave and Houdini