I'm actually quite knowledgable with psitticines. How old are these birds? A mature male will have a blue/purpleish cere and a female will have a peach cere. If a female is in estrus the cere will be brown & crusty looking. Immature birds all have a peach cere. I can't tell the exact cere color from this picture. It looks like the yellow budgie might have a blue cere but it is hard to tell. Sometimes it is really hard to tell on these color mutations. I hope this helps.