Once a year, Humboldt penguins change all their feathers. Here with us, this happens in late summer/early fall - around August and September. Basically, they are white on the front of the body with black dots on the belly, while the back is completely black. This is a very common style of camouflage in the wild, with many animals being lighter on the belly than on the back. Towards summer, the feathers on the penguin's back temporarily start to become more and more brownish in color, and this is a sure sign that they are getting ready to change their plumage. When they are ready, the old feathers will stand out from the body, making them look quite fluffy, and little by little they will fall off. During this time the penguins won't be in the water much and their appetite will drop, but luckily it only lasts for a few weeks and they will quickly settle into their new plumage.
The penguin chicks also go through feather changes. One when they are newly hatched, and one when they are just over 1 year old. Then they will go from having the chick plumage to the adult plumage, and get the characteristic dark stripe across the chest and a darker color on the face.