well, the green seeds are more likely to have a strong flavor of cilantro along with the orange-like flavor of the coriander seed, making your witbier taste a little 'off'. If you pick them before they turn brown they may not be fully developed and you may have storage issues. While they are still quite young, green and soft, they can be pickled - kinda like capers. For the 'classic' flavor, let them turn brown, cut the seed-producing stalks, let them dry for a couple days, shake them off, roll them on a cloth to get the dirt and dried husks off and then use them or store them. Either way, fresh from your own garden will taste much more vibrant than from the store.
also: if you try to grow from the superior-tasting coriander seeds from India, be aware that your soil and growing conditions are different, and the result will be different, but still better that the 'domestic' seeds.