Hi,
If the cere is a bright/deep true blue, then you have a male, and it is probably older than 6 months. It can take up to a year for the cere color to change, but a bright/deep true blue indicates a mature male. If the cere is a pale lavender then it could be male or female. Cere color is not an indication of gender in all of the color mutations of parakeets. In the pastels and solid color color mutations, the cere will be a pinkish lavender, and will remain that color regardless of the age or gender. With these colors, it is wise to have the DNA testing done. The standard blue and standard grey budgies will have the true blue cere for a male, and white to brown for a female. But there are pastel and solids of these colors, and these would require a DNA test to be sure of the gender.
I will say that breeders often have insight that might go against what you will learn by research. This would usually be a breeder who breeds for quality birds, where they only have one pair per cage and rest their pairs between clutches. This is the responsible way to breed parakeets, and will result in healthier, robust offspring. Unfortunately, most volume breeders will tell you whatever they think you want to hear. This would be the breeder who has dozens of birds in each flight, with the flight lined with nest boxes. They keep no records and have no control over which birds breed with each other, so a lot of inbreeding takes place(the breeding of related birds). They allow year round breeding, and have no problem allowing birds to breed until it kills them. The birds are genetically flawed – usually very pretty, but tend to have very short lifespans.
For the best results with breeding, wait until the birds are 2 years old, only have one pair per cage, never allow related birds to breed and rest your pairs for 6 months in between each clutch. And of course feed a quality diet of pellets, greens, veggies and during breeding, an egg food. Each pair should also have a cuttlebone and/or mineral block. A foraging pellet such as Nutri-Berries or Avi-Cakes can be fed instead of pellets, and will provide some fresh, whole seeds while still being nutritionally balanced.
Thank you for asking Lafeber,
Brenda