What colors on a budgies beak mean it is a male or female?

It is typical for adult make budgies to have a blue cere, while adult female bugies will have a brown cere. For some genetic mutations in companion budgies, the cere color does not change, such as in albino, dark eyed clear and lutino budgies. Immature budgies will usually have a pink cere whether they are male or female.

To find out more information on how to tell if your pet budgie is a male or a female, check out this FAQ article all about Budgerigars which explains it in more detail:

http://theparrotforum.com/viewtopic.php?f=12&t=355

The article also explains more about the characteristics of Budgies and how to care for them.
In general, male keets have blue ceres and females have white/brown/beige/tan. The cere is the colored area above the beak. However, the cere has to be fully colored out (adult coloration) or the cere color may change with the first and/or second molts, depending on the overall coloration of the bird. Additional information about various color mutations/cere color can be found on the internet.