If you drag from material window of interface to canvas it turns to flat material - this is useful to check silhouette . As long as you haven’t filled the flat material to a subtool just reselecting any shaded material you wish will return the subtool out of the flat color material to that selected.
For reference also : to remove material information from a subtool but keep rgb / polypaint information> fill with flat color material >choose a new material to view your rgb information
You can use subtool master to fill every subtool layer this way.
Hope that is of some help.