The Great Pyrenees reputation as big, white dogs has caused some confusion when people see Pyrs with darker colors throughout their coats. I attended an AKC show once, where all the Pyrs were pure white. (The AKC defines the Pyr color as "a white or principally white coat that may contain markings of badger, gray, or varying shades of tan.")
In our rescue days, people were quick to dispute a certain Pyr or two of ours as not being "full" Pyrs, due to some of the color variations they'd see. In addition to the potential for badger (the darker trimmed colors around the ears), gray, and tan, I've also read that it is acceptable to see colors more reddish in appearance. Such was the case with a couple of Pyrs I've met.
Pyrs can have any or all of these other colors anywhere on their body! The only general rule of thumb as a disqualifier is if these darker colorations cover more than 1/3 of their total body. So your true Pyr could have a white head, with dark colors throughout. They could have a very dark, masked face and a white body. Any combination is okay! My Ana had a very dark grey mask as a puppy; people thought she was a mix. These days, it looks more black than grey. Christie's Reg has colors which can only be described as more caramel in color, and they are seen frequently throughout his coat.
Naturally, it is also quite common to have a litter of pups with all different variations. All of Ana's siblings were pure white! If you are in the market to adopt a Pyrenees and really want to get one you perceive to be pure, don't be concerned about some of the fun colors you will see!