Dear jezi prty:
What colour would your cockatiel be? Lemon yellow maybe?
Is the bald spot behind her crest on the back of her head?
Her color is Lemon yellow. The bald spot is behind her crest on the back of her head.
As I suspected, you have a Lutino cockatiel, one of the most beatiful colors mutations. While the breeders were selecting for this color, they bred the 'bald spot' into the coloration.
There is absolutely nothing wrong with your bird. This is a normal trait that goes with the Lutino color
I thought you might be interested in this link which has some interesting information regarding the color mutations: http://www.cockatielfoundation.com/color-mutation-center.html.
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Good-luck with your beautiful girl.