I have grown both green and purple asparagus. The purple has a milder flavor than the green. The flavor is more similar to the white. Any type can be blanched or hilled to produce the white spears. You are basically just keeping the plant from producing any chlorophyll or other plant pigments by keeping it growing in the dark. Same principle as growing Belgiun endive. (nice to know my degree is useful sometimes, lol)

The growing methods of purple and green vs. white are different which would account for some difference of flavor. White asparagus is "hilled" (has dirt banked up against it as it grows) so the green color won't develop which occurs naturally as the plant is exposed to the sun. Green and purple asparagus just sprout naturally from the roots and aren't "hilled". And the purple asparagus will turn green in cooking, just like purple string beans and purple okra. These plants come from variations in breeding, whereas white asparagus comes from different cultivating practices. Hope this makes sense.

In addition to being a little sweeter and more bitter, the flavors in white asparagus are also overall subtler - less of that springy grassy flavor you get from green asparagus. Purple is much more like green, though I find it to be just slightly more bitter.

I haven't noticed a flavor difference between green and purple asparagus, but white asparagus has a bit more sweetness and bitterness (in a good way).