You may want to get a cheap spray can holder like below and modify it to work with a servo.
https://www.google.com/search?q=can+spray+paint+gun&num=100&hl=en&lr=&newwindow=1&safe=off&as_qdr=all&prmd=imvns&source=univ&tbm=shop&tbo=u&sa=X&ei=4LMuUNizGdStyAH7p4DoDw&ved=0CIwFELMY&biw=1291&bih=627