They are using two marigolds that, when crossed, the resulting seed consistently produces white flowering marigolds. Its an unstable hybrid so to get the seed, they always have to go back to that hybrid cross. I am surprised the seed from white marigolds is even viable. Its done with corn and tomatoes all the time.
I bet once in awhile, there is an outlier in the package.
You could grow white marigolds and collect the seed. Keep planting and collecting seed and planting. Somewhere, eventually, you will get a white marigold. Then you just have to know which of your many offspring plants produced the white marigold. Good luck with that...
Daisy
