When hiring a contractor it is always best to hire one who is personally referred to you by someone you trust. Hiring through ads or phone book is hit and miss. Check with your local paints stores, they know the good guys from the bottom feeders. Go to the stores that sell the high quality paints like Benjamin Moore(nonpareil), Pittsburg, Sherwin Williams or Glidden. Don't go the the big box stores for referrals, the people there don't know squat!
I used to work at Sherwin Williams. The best thing to do is specify that you will purchase your own paint. If the painter objects strenuously, he was plaanning on making money in the ways this article outlines. You might pay a few dollars more for paint, but you will have control of the quality of what goes on your walls. Never skimp on paint quality.