I swear it's all related to level. The higher tier level you are, the more likely better color drops appear. Under lvl 20 you have such a slim chance for a random blue and purple. 20-23 most drops are blue with a small chance of purple. 24+ there's a higher chance of purple and a small chance of yellow. The quickest way is to not worry about the drops and focus on light gear. I spent so much time farming from 20-23 and got zilch. Now that I'm 24 I get a few purples every hour.