I am a painting contractor and have been since 2001. Make sure the estimate provides in writing: What is EXCLUDED as well as INCLUDED. It should state the manufacturer and type of paint going to be used. Estimate says ALL LABOR AND MATERIALS. My estimates to my customers say "guaranteed coverage" eliminates the conversations of 1 coat vs 2 coat. I have my customers submit colors 5 days prior to start date. Customers need to inform me if they are going to use pure white, dark reds, oranges, and bright yellows they need to inform me in that 5 day window, so I can adjust my pricing for 3 coats. Although this more uncommon now than years past because a lot of paint manufacturers have primer with paint products. Let the contractor know if your doing accent walls. This takes longer to cut in straight lines and it requires the contractor to purchase more paint. If you add anything on the scope of work have the painter write out the description and cost prior to them doing the work. Have the estimate say how many days it will take to perform the work. Ask how many workers will be doing the job. Make sure to enforce that number of workers their everyday until the job is complete. Do not give final payment until you do a final walk through. Walk the job when its almost complete and point out areas that you want fixed prior to the contractors final walk through. Its best to do while the workers are still in that particular area as they will have tarps down and areas covered and it will be easier for them to take care of. Purchase a roll of blue tape and stick it to areas that you want fixed. This is called a punch list.
If your home was built before 1979, the paint might contain lead, which is extremely hazardous to children age 6 and younger. Call your local public health department for information on how to check for lead and handle lead paint safely. For a free booklet on dealing with lead paint, contact the U.S. Environmental Protection Agency at www.epa.gov/lead.
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.