Dark grey with yellowish heads sounds like a normal grey cockatiel. I have one of those. Males have bright yellow heads when mature, females a washed yellow/grey head when mature. Greenish??? there is no such thing in a cockatiel so not sure what she means.

Most books these days pretty much list everything you need to know. Internet is also good for searching about them. You can check out my 1 page on cockatiels here:
http://eclectusparrots.net/cockatiels.html
It has a little basic info on cockatiels that isn't often brought up anywhere else.

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