How to get WireColor Element with all different colors?

After rendered, the VRayWireColor Element results like this:

This happens becasue all these objects, have their WireColor set to black:

To solve this issue select every object and change this little coloured box on the left “name and color”.
Do this for every object and this will be the result after:

Why this happens?
This could happens with imported objects, conversely objects created in 3ds Max have automatically different colors assigned.