Here is where this affects you as a consumer. You select a painter with a contract that says 2 coats, $500 down. You give the company the deposit and pick your colors a couple of days before the project starts. The painter goes to the store with your colors and figures out they are deep base. He (or she) not only needs to charge you more for the paint, but he also needs to charge you for a dark gray primer coat. Ninety nine percent of the time that primer coat is going to be really, really expensive since you already gave a deposit.
Second coats on similar colors are almost never recogicnized as being needed until the coat is applied and has dried. ONLY THEN WILL YOU SEE WHETHER IT NEEDS A SECOND COAT or not. Yes, painters can use a cheaper paint then what you paid for. That is solved by getting your own which, I would charge extra for because I will always have to go get more, or add second coat because home owner tried to skimp on paint, or they got the wrong color etc...
I do not mean to come off with an edge, however, this is an article for people hiring low bid, uninsured, one man band painters out to "hustle" a buck. Hire reputable, established companies or hire someone you know or have a strong referral from. If you are arguing over watered down paint, ticky, tacky deposits, buying your own supplies, you have hired the wrong guy.
The Lead Renovation, Repair and Painting (RRP) Rule establishes requirements for firms and individuals performing renovations, and affects contractors, property managers, and others who disturb painted surfaces. It applies to work in houses, apartments, and child-occupied facilities (such as schools and day-care centers) built before 1978. It includes pre-renovation education requirements as well as training, firm certification, and work practice requirements.