That discoloration is a porphyrin staining. Dogs have this stain in their saliva normally. Curly is obviously licking his paws most likely due to an underlying allergy. Seasonal and non-seasonal environmental and food allergies can cause a pet to lick or chew on the feet, knees, or other body parts. Inflammation can also be caused by items embedded in the skin, painful joints, flea bites, etc.
The staining around the mouth is most likely due to a dental infection. More saliva is produced in attempt to rid the bacteria from being absorbed through the gums into the bloodstream.
I would recommend you try this shampoo:
https://www.target.com/p/veterinary-formula-clinical-care-hot-spot-itch-relief-pet-shampoo-16-oz/-/A-50298965#lnk=sametabReally though, Curly should have a vet exam so his mouth can be evaluated and the allergies can be addressed (if it is allergy related).
I hope this helps. Best wishes.