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.
When the homeowner is at the point of hiring a painter, they generally will have colors selected or at the very least a color in mind. I always ask for the colors before I bid a job. Dark colors, high sheen colors and specialty finishes require more labor, this drives price. If its not a color change or I'm going over a similar color I give the pricing option of one or two coats. The best advise I can give based on 25 years in the business is to put it all in writing,colors, brands of paint preferred, when the work can be done, who moves furniture and how payment will be handled. I never get up front money. BTW you most definitely get what you pay for with paint. Higher quality products results in and better looking job. Don't be a cheapskate when it comes to paint or the painting contractor.
There are permanent features of the home that have their own colors which cannot be changed when painting the exterior of the home but can have a dramatic satisfaction level that is experienced when the painting is completed. Roofing shingles, paving blocks, concrete surfaces, stones and other such features are prime examples of the colors that should be considered when selecting your exterior color scheme for the home. Homeowners can work off of these colors to make a home that looks uniform and luxurious when the exterior painting is complete.