Back to the article. You can add water to all latex based paints / thinner to oil based paint. The tinting base has absolutely nothing to do with it. Say you are working outside and throughout the day you have to add a little water to keep the same consistency. If somebody really tried to add 20% to 50% water they no longer would be painting they'd be performing a whitewash or pickle finish.
More recently, professional painters are responsible for all preparation prior to painting. All scraping, sanding, wallpaper removal, caulking, drywall or wood repair, patching, stain removal, filling nail holes or any defects with plaster or putty, cleaning, taping, preparation and priming are considered to be done by the professional contracted painter.
to be the devils advocate i have been a building contractor 20 of the last 30 yrs. i do know that if you go to a higher sheen of paint and or darker colors then any imperfections in the walls will show up much more dramatically…therefore the painter or a good drywall finisher is needed to prepare the walls extensively. this could cause more expenses…for it takes a lot of time to prep walls (smooth walls..not textured walls) and this cost has to be absorbed.
I used to work at Sherwin Williams. The best thing to do is specify that you will purchase your own paint. If the painter objects strenuously, he was plaanning on making money in the ways this article outlines. You might pay a few dollars more for paint, but you will have control of the quality of what goes on your walls. Never skimp on paint quality.