I used to think that all parakeets were either blue, yellow or green. But when my uncle started running a pet shop, I saw so many different parakeet colors there. There are actually many color mutations for parakeets but some are just not as common as others. Even the blue and green ones come in many different shades depending on the base color.
I had the opportunity to speak with someone who breeds parakeets for the pet shop and he told me that parakeets have a base color, a body color and then stripes. The base color is the color of the face, body color is different and a variety of tones can emerge when body and base color mix. The stripes are usually gray or black.
There are even albino parakeets that only have a base color like white or yellow. It's quite interesting.