Second: all the tricks of the trade in regard to "cheating" customers is for hustlers and cheaters and NOT established businesses. At the end of the project the job should come out looking professionally painted as specified in the contract. A selected color that takes multiple coats that was not calculated by the contractor should cost more money. It's not the fault of the painter.
Safest way to ensure that everything is fair is to get it ALL in writing , signed by both parties. Specify each item that needs repair. Also, BUY the paint YOURSELF. That way, there is no incentive to water it down, and you KNOW that you are getting the grade/quality you actually purchased. Don't be penny wise and pound foolish; if you are paying to hire a painter, buy the best paint that you can afford, to ensure maximum life of this home improvement.
When you move from room to room, consider your color story, or how the colors will interact. Using complementary colors - colors that sit opposite one another on the color wheel, will create a very dramatic look in your home. Choosing analogous colors, which sit beside one another on the color wheel, creates a more subtle appearance. Monochromatic looks can also be very effective in small spaces, when you choose varying shades of one color to give variety, but without definition.
Good contracts include descriptions of prep work and repairs; paint specs by brand name, type, color and product number; the number of coats; and a full description of the work, including frequently omitted items such as cabinet interiors and shutters. Minimize delays by specifying that, weather permitting, work will be continuous. Get a payment schedule that minimizes the down payment — the more payment you can withhold until the end, the more leverage you'll have to get the job done well and per your specifications. Insist that contractors provide proof that they carry both general liability and workers' compensation coverage.
I agree with JHs. If there is no new color on it at all, just talk to the painter about it, if it was an honest mistake (which can happen very easily while painting) your painter should have no problem fixing it. However, in my years of experience, it is not unusual for 2 coats of door paint (good quality) not to cover very well at all. I once painted a red door 7 times, plus a tinted prime coat before I found the door to be a solid color.