female normal grey's faces are dull and males' are bright. there's other things too but I noticed you said you have one with a grey face and one with a yellow face. the one with the grey face is most likely the female.

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I have to normal greys cockatiels I never see them mate but one lays eggs and they help with the nest I put a fertile egg under them and it hatched and they help each other feed it one has a grey face the other has a yellow face but they both have dull orange patches on there face the tails are both the same witch has strips on them the female stands in front of the other one and put her but on it and moves back and forth and making a soft chirping noise and it just walks away and a few minutes later that one does the same to it