Actually, Disneygirlatheart, what you probably have is not a white poodle, but one of the other colors that occur but are not recognized by the AKC.
These colors pop up occasionally, more so in back yard breeder litters and puppy mill pups - people who don't have scrupulous breeding ethics. However, even the best breeder has had the "old" colors pop up in their litters. You cannot breed the genetics out totally, they still pop up once in awhile.
Without seeing the dog, your description sounds like you might have a brindle and cream parti-colored dog. The cream being the "yellowish color" you see and the brindle parti - the muddy colors on the feet and face - the common places to see parti-colors.
For me, it would not be a matter of a fungus or an allergy (shouldn't diagnose medical conditions without seeing a dog) because you said the hair always grows back the same color. This sounds like this is actually the dogs color and not a disease. A fungus or allergy would not make the dog a parti-colored dog (ie, darker colored feet). You would also be battling skin problems. If you do have concerns about a fungus or allergy, please take your dog to the vet to get an expert's opinion.
Here is a website showing some of the "off colors" of poodles.
http://www.homestead.com/sharminspoodles/ToyPoodleColors.html