A friend of mine from Wisconsin who sells asparagus at a local farmers' market told me how he "creates" white asparagus. The actual plant is the same as the regular green. For a long time, the farmers created hills to keep the asparagus spears buried as they grew - this protected them from sunlight and they did not turn green. The downside was the labor involved and trying to harvest below ground - many spears were damaged and harvesters were injured with the sharp knives. So, now the farmers use dark poly to protect the young spears from sunlight as they emerge from the soil. They can then be harvested above the soil and less are lost. It is still a labor and time intensive process and makes the white version much more expensive than the green.
Since my palette is not refined enough to make the difference significant, I just enjoy the green asparagus in every way possible!
Cheers,
David