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.
When coverage is difficult to estimate, add more rather than less when doing DIY wall painting. You can always pour the leftover back into cans. For large jobs, use the bucket and a roller screen rather than a roller tray. It’s much faster to load your roller with the screen than to use a roller pan. Simply dunk the roller into the paint bucket, then roll it along the screen until it stops dripping.
This all comes down to the rules.....1. references....does the contractor have them??? I ALWAYS furnish all my prospective customers them....no excuses...2. insurance....again, I always furnish proof....3. Read the proposal carefully...I ALWAYS list materials down to tape used, the brand, the grit of sandpaper, the manufacturer, etc....its INEXCUSABLE to not list all of these items....I am a member of the PDCA, the Painting and Decorating Contractors of America, the foremost authority in the coatings industry and they also approve of what I listed....if you do not follow these guidelines, you will NOT get a job reflective of "professional". Look for the PDCA where any painting contractors are, if they are not a member, RUN!
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.
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.