Most removable discoloration is caused by airborne particles and dust.
If a cleaning with a soft cloth and mild soapy water doesn't clean it. STOP!
And consult the Mask Doctor.
If the problem is a stain, chances are the mask will need to be repainted.
Common stains are from fake blood, paint, or clothing dye.
Other causes of mask discoloration are,
Sun darkening, but by this time the mask is usually too brittle to save!
Carbon from smoke damage.
In rare cases,"mask rust", which colors the mask an orange-yellow.
All of these are fairly serious, and you would be well advised to contact the Mask Doctor for additional help.