Hi there! I’m fairly new to ZBrush, so I’ll be asking a lot of questions. I was creating a ZSphere sculpture, and pressed a to see how it would look. Unfortunately, the whole thing was white, and didn’t have a clay-like appearance. I don’t know what I did wrong, but I hope that someone can explain this thing to me. Please help! Thank you 
Are you still in Render >> Preview mode? It sounds like you might have accidentally turned on Flat mode instead.
Thank you! Your advice fixed my problem