If the polygon boundaries fit with the edges you want you could select the to-be-white polygons with the Geometry Editor tool, in whole or in part, and then right-click and select Geometry Assignment>Create Surface from Selected. If you selected only some polygons initially you can select more, right-click, Geometry Assignment>Assign to Surface>Surface name. Once you have your surface, use the Surfaces pane to edit its settings.

If the boundaries don't work then another option is to create an image that is white where you want to cloth effect, black elsewhere, and then from the image assignment menu select Layered Image editor, create a new solid colour layer of the desired shade, and add the image as a mask.