Lastly, chose a sheen to match the needs of the space. Flat finishes hide imperfections well and absorb light which can make the paint look darker. Eggshells and satins are tough enough to be used in family rooms, kids' rooms, and hallways but don’t hide surface imperfection well. Semi-Gloss and high-gloss finishes are primarily used for trim because they accentuate woodwork details and clean up easily.
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.