Don’t know if you’re still having problems, but you can do the following: Make sure you are in Sculpt Mode
1. Press Alt+M or Click Mask->Clear Mask.
2. Press Alt+H or Click Sculpt->Show All.
3. Press TAB or Switch to edit mode.
4. Press A or Click Select->All.
5. Press TAB or Switch back to Sculpt mode.
6. Click Face Sets->Face Set from edit mode selection. Mesh will change to 1 color.
7. Click Face Sets->Face Set from Visible. Mesh will change to grey color.