FarmViller gives a table of probabilities, check there
It seems that the white egg give 30% chance on a brown chicken.
The brown egg gives 30% chance on a black chicken
the black egg gives 30% chance on a golden chicken
the golden egg gives uhm pretty much the contents of mystery box I :)
I will add this too to the database as soon as I came to it.
(found via FarmVille Fanatic)
(and some threads on the forum:
- http://forums.zynga.com/showthread.php?t=260850
- http://forums.zynga.com/showthread.php?t=260797
- http://forums.zynga.com/showthread.php?t=262468