Some specific epithets have an historic meaning while others were just given a name. Prefixes are numerous both in the Latin and the Greek languages. The prefix changes the meaning from purple to dark purple. New epithets are invented from time to time, and their meanings require a little guessing if you cannot find them in the references. This plant does indeed come from the mountains of Southwestern USA, notably from the States of New Mexico and Arizona.
Suffixes permit a wide variety of words to be formed from a single word.
It does help to know the meaning of the most common prefixes and suffixes because they will help you to understand the meaning of many specific epithets without the need to look them up. Because nouns in Latin change form to express relations or different genders, its following descriptive adjective may or may not reflect the gender and the declension of the noun it describes. Annuus, annua, and annuum all have exactly the same meaning, which reflect the gender of nouns. If by now you are totally confused, do not worry too much about the issue. Treat all botanical binomials as English words.