The KiCad 3D Viewer has seen a few incremental improvements during the course of V6 development:
Plated and Non-Plated Copper
A subtle change has been made in MR#405 by Mario Luzeiro that affects how copper is rendered. The visual difference between plated copper pads and non-plated copper pads will now be visible as well as copper in general.
This image shows a ENIG plated copper hole compared to the surrounding copper traces when the soldermask was turned off.