You could do it using the Flat Color material for the other SubTools:
- For the Fibers, fill with a suitable material.
- For all the other SubTools, make sure Colorize is off in the Tool > Polypaint sub-palette.
- Select the Flat Color material and white in the color picker.
If you render with BPR then all the SubTools will render as falt white. You could set the Document > Back to white also, and get rid of the gradient, if you needed to but the Mask and Shaded images in the RenderPass sub-palette should make it easy to isolate the fibers.
HTH,