I don’t know what happened, all of a sudden every subtool turned gray. It kind of looks like when you switch to the Lightning View in Maya but you haven’t created any lights, so I opened the Light Menu, but the default light is on. It’s not the material either, I’ve tried several other with no different result. Masking and unmasking aren’t solving the problem either. I don’t know what else to do…
First off, why are you using ZBrush 4R6?
https://pixologic.com/zbrush/downloadcenter/instructions/full-installer.php
Check to make sure the Render mode isn’t set to flat.
Also exit edit mode, and make sure MRGB and Z intensity are both set to 100% before drawing it onto the document. If its RGB only then the model wont be drawn with any material regardless if what you try to fill it with later, and if its M only then the model will be stuck on grayscale. MRGB will allow the model to have both.