Polymorphism - a breeder's nightmare.
Basically, if an ancestor had a particular colour or pattern, you can expect it to show up again, no matter what the current parents look like. You can increase the odds of getting a particular colour by line breeding, but it's still not guaranteed. It would take many generations to 'breed out' the other colours and patterns, but it is possible. Since cresties have only been bred in captivity for a relatively short time, it's not yet been refined to the point where we can guarantee what the offspring will look like.
To give you an idea of how unpredictable Polymorphism can be, here's a chart I made up last year. I've always meant to add more offpsring pictures, but never got around to it. There is enough though to show what can happen.