HOW TO TELL THE DIFFERENCE BETWEEN MALE AND FEMALE BUDGIE - BABIES AND ADULTS
Right above budgie's beak there is a part called cere. Luckily for us - males and females ceres are different colours. However - baby budgies, or young budgies ceres are different than mature birds'. A male has purple/pink cere (more or less purple or pink). A female's cere is lighter, pale pink with white around nostrils. The difference is visible as early as around the 3rd week of budgie's life. And of course it's easier to spot the difference if you have a couple or more baby budgies together, so you can easily compare their ceres. Female's cere will always be much lighter than male's.
Above there are female ceres. They're either few weeks old babies, or young females under 6 months old. Firstly pinkish/whitish cere then becomes white, with a hint of powder blue.
Male ceres are darker and deeper in colour than females. They're purple with no white around nostrils. Above - some pictures of male baby budgies - from a couple of weeks to few months of age.
When it comes to determining adult budgies sex - it's gets much easier. Kind of..
We can take as a rule that adult male's cere is deep/royal blue. Adult female's cere is normally white or pale/powdery blue. But there is an important detail to note. Whilst male cere stays blue in all conditions (considering healthy bird) - female's cere turns brown when she comes to breeding condition. It's initially beige/cream, then it gets darker until it becomes chocolate brown. It also gets thick and crusty, sometimes looking like it's overgrown. it's also good to keep in mind that some females have white cere even if in breeding condition. White cere doesn't always mean she's not interested in breeding, but it's less likely.
Above a blue female budgie out of breeding condition (look at her cere - it's pale blue/white)
Above - green male (dark blue cere) and yellow female (dark brown cere - breeding condition)
Above - young blue female (around 4-6 months old) with nearly white cere
Above - adult blue male budgie. Dark blue cere
Above - 2 months old yellow head baby budgie - male (dark pink/purple cere)
The picture above shows a young female budgie. Her cere is light bluish/white with hint of light brown, as she's just about to start to come into breeding condition.
Above you can see how this female's cere turned from light brown to chocolate brown. Very dark brown and crusty cere also means female has passed her peak and is about to start to come out of condition.
Above - yellow female budgie and the difference between her cere in and out of condition.
It's important to remember that although blue cere generally means male - it matters what shade of blue it is. Note - pale blue: female. Dark blue - male.
However - there is an exception from the rule about male blue cere. Some mutations, like recessive pieds don't have blue ceres. Their cere stays purple and never changes colour. Note: it applies to males only.
Above - recessive pied - adult male with his purple cere.