coat getting darker in certain spots

i have a 6 year old cocker spaniel. her coat has been a golden light brown her whole life.. all of a sudden the coat on her feet (between toes) and around her mouth has darkened considerably to a dark bronze colour.. the strange thing is it gets lighter and darker... can this be an allergy?

Our vet said that a dog's saliva can darken the fur. Has she been nibbling or washing her toes a lot? That's what Ebony does and it turned the white hair between her toes to brown.

~LM~