When mildew developed inside my accordion case I dusted the velvet cloth inside with dry borax powder and placed it it the sun.

Hydrogen Peroxide is a fine bleach.

Sincerely,
Gargoyle

Stains can make for an interesting pattern, Keep then, or accentuate with a another stain such as a random scattering of moist yellow onion skins ( that are removed after an hour or so)