Whoever told you the budgie's gender can only tell by the cere, they did not use any other means to judge your birds gender. They would have had to see from the picture you provided. If the cere looks different in person than in the pictures, then it makes it harder to give an accurate guess.
A blue cere, with white around the nares indicates a female, if in a young hen that isn't in breeding condition.
If you can please provide a recent picture in good lighting without the use of a flash, we shall be able to tell.