As an owners rep and former estimator for a gc i do expect to pay a reasonable deposit say 10 percent and then weekly progress payments. I am asking the painter to book his time and to do the work on my schedule. Most painting companies are not huge operations they will pay their staff weekly. For most repaints i ask for one tinted primer coat and one finish coat for 100 percent coverage. New work gets one tinted primer and two finish coats for 100 percent coverage. The cost of the paint is not really that different for colors or finish. Brands like european fine paint and C2 are more expensive than Ben Moore and sometimes more difficult to roll based on their consistency. Some home depot brands are as expensive as the Ben Moore and of equal quality others aren't. Red paints are notorious for number of coats required and special priming requirements.
Oh, where to begin? Let me start with 'watered down paint'. 25-50% before the material gets to the site? Impossible. You would basically be painting with water at that point. It would be less of a hassle, and cost, to simply use proper material. You would be forced to apply three coats instead of two, as the coverage would be horrible. Whatever cost you think might be saved in materials would be lost in labor.