Vertex maps are only available for meshes and created automatically during the meshing process. These maps contain channel information with the fluid's velocity, vorticity, age, and weight.
The creation and properties of the vertex maps can be controlled through the "Mesher" object's → "Channels" tab.
Weight Maps
Weight maps are used for blending effects between differently coloured fluids or emitters. RealFlow | Cinema 4D's "Mesher" object creates one vertex map per fluid/emitter, based on the distance between particles. As long as the particles of the different fluids/emitters are separated each node will have its own colour. As soon as the interaction starts the colours will be blended and you will see the mixing effect.
The usage of vertex maps requires a certain → workflow.