Why does my purple shirt appear as blue on my iPhone 4?

Whenever i take a picture with my iphone 4 and my purple shirt is on, it appears as blue

Because iPhones suck!

Because it is primarily a phone, not a camera.

Probably because of the light or if it's too dark then it would appear blue as well.

Did you try wearing a purple shirt and then taking a picture without AE locking?