If the point of hiring a well established, experienced, reputable painting contractor is to secure the professionalism and trust suggested to be inherent with that choice, then I would EXPECT that professionalism and experience to include the ability to make the proper and correct calculations for labor and materials for a fixed price quote, and there should be NO reason for the contractor to put the cost burden of their miscalculation on the consumer.
Never, never hire "Hanks painting" , he is a scam. He charged me 1800 for an 800 sqft house, with the agreement of painting the gutters. He not only didn't paint the gutters, the paint of the walls and front door is pealing off. He did not sand the door and the wall color is washing off. His people don't know the work and he much less than them since he painted the door.
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.
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.
Generally, it will take a minimum of two gallons of paint to cover a room. At the highest end, paint will cost anywhere between $30 and $60 per gallon and come in three different finishes: flat, semi-gloss or high-gloss. Flat finishes are the least shiny and are best suited for areas requiring frequent cleaning. Semi-gloss finishes are a bit shiny but can also be easily cleaned. A high-gloss finish is stain-resistant and easy to clean. Traditionally, living rooms should be painted with a flat finish to allow the paint to stand out. Glossy finishes should be reserved for hallways, and a semi-gloss is best suited for trim. Invest in pre-primed paint whenever possible. This cuts down on time by combining the layering process. Almost any paint job will require a primer, which will cost anywhere from $7 to $15 per can. Primer helps the paint to stand out against the underlayer of paint it's covering, especially if the new paint is lighter than the old coat. You will need at least two cans of primer, if not more, to cover one wall. Here are some paint brands and their average price per gallon:
Some proposals simply say to paint the walls and ceiling and never specify the number of coats to be applied. If the colors are similar enough, it's possible to get away with one coat of paint and not discount your pricing. No matter how hard you try, tiny, pin-sized air holes will pop exposing the original walls. This may not bother you if you can't notice it, but principally speaking you should have paid your painter less for the work.