Noses can peel and flake much like our skin does, especially if they have been exposed to sun, rain, wind, or trauma. Under neath the normally black nose is pink skin. If some of the black comes of for any reason, you will see the pink.
If this happens a lot, you will want to see your vet to rule out a thyroid condition that might be the cause.
While the skin is pink, be sure to use sunblock on the nose when the dog goes outside. You can also use "bag balm" to lubricate the nose and keep it soft and prevent this in the future. You can get it at walmart.
Let me know if you have more questions.