Google is your friend. Chances are good if it was purple it was locally sourced.
The main differences with white and purple garlic are in their availability and effect in cooking. White garlic is more prevalent. Softneck garlic, which are always white, are easier to raise and are hardier (despite what the name might imply). They are more conducive to shipping and travel. They tend to have a very powerful scent and flavor. Often, white garlic has more cloves than purple garlic.
Purple garlic varieties are under the ophioscorodon varieties aka ‘hardneck.’ They, and their ilk like porcelain and rocambole types, are referred to as such because of the snake-like stalk which coils when they grow. By comparison to white garlic, purple garlic has fewer cloves but the bulbs are generally larger. Purple garlic, or more accurately referred to as purple-striped garlic, have several varieties as well, such as Persian Star, Chesnok Red, Metechi and Persian Star, among others. Interestingly, most of the common purple garlic originate from the former Soviet Union. Compared to the white varieties, purple-striped garlic has a milder odor and taste; the Chesnok Red, for instance, even has a sweet tange to it. Purple Garlic do not store as well as white garlic but retain their taste longer even after cooking.