I disagree with your criteria to hire a painter. A prompt returned call is nice but does not indicate the quality or fairness of the painter. As far as a written estimate, that should be more of a qualifier for the bid versus an evaluation criterion. I'm not sure one would have favorable results by hiring a painter on this basis. As far as the bidding process, change orders should ONLY be used if the customer requests additional scope (PMP 101). Angie's list should consider asking reviewers if/how much they were told to pay compared with the estimate. Unfortunately, there are a lot of contractors that are unethical and need to be accountable.
Historically, the painter was responsible for the mixing of the paint; keeping a ready supply of pigments, oils, thinners and driers. The painter would use his experience to determine a suitable mixture depending on the nature of the job. In modern times, the painter is primarily responsible for preparation of the surface to be painted, such as patching holes in drywall, using masking tape and other protection on surfaces not to be painted, applying the paint and then cleaning up.
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.
Thanks, all, for your time & efforts adding to the article & comments, especially Dave urging requesting both General Liability AND Worker Compensation insurance certificates to protect from real & fraudulent liability--from my experience especially in California, where insurance fraud is a popular income thief, even causing car collisions to collect.