I am having a problem with Zbrush while trying some of the hotkeys of brushes, I have surprised with flat view of the model as appears I the attached image. I have tried to change the silhouette and give a new material but the problem still persists.
Im guessing what the problem is but probably you applied a color and a material to the model, thats why you cant change the material , you could go to the right menu, under Polypaint , try turning off the colorize button
top middles you have mrgb and M
make sure one of those are on when you fill color or material,
it maybe the flat material doing it but it all so looks like you flatten the UV out all so
uvmaster>unflatten
or
tool>uv map>morph uv