Color is the first thing most people think about in selecting paint. Yet color should be the last concern. It is more important to choose the correct paint for the job.
For both interior and exterior applications, there are many paints to choose from: acrylic-based, oil-based, one-coat, two-coat, under-coat, over-coat, trench-coat, expensive, cheap and any combination of these.
ACRYLIC VS. OIL
There is some debate over the relative benefits of oil and acrylic-based paints. Acrylic (or latex) is easier to use, easier to clean out of a brush, and easier to wash out of your wife's hair if you happen to splatter her while you are working above her on a ladder. Oil-based paints have a reputation for lasting longer between coats.
I have never personally tested the two different paints side by side. Cleanup with oil does not need to be difficult. Some people use mineral spirits or even gasoline as a solvent. Just don't use gas around an open flame (or even an unopen flame). The easiest cleanup method involves simply throwing away your brushes, paint pans and rollers at the end of each job. It can be a costly way to clean, but it does save time. You may also wrap a used brush in plastic and stick it in the freezer. The brush will not dry out and will be ready for use another day. Just don't forget about it and pull it out two months later thinking it is one of those blue gills you caught during an expedition to Uncle Ed's fishing shack at Lake Wanka-Tonka-Hullaballoo. Oil brushes don't cook up all that well.
As for your wife's hair, bleach will not remove oil-based paint. Nevertheless, if you have always wanted to see her as a blonde, you can tell her that it is the ONLY thing that will remove the paint. She will look like a different woman (and after she finds out you lied to her, she will act like one too).
ONE COAT VS TWO COAT
There is not a paint sold in America that does not proclaim on the can its ability to apply in one coat. Beware: many one-coat paints are only one-coat paints if they are applied over the same color. The difference between a true one-coat paint and any other would theoretically be in its thickness.
There is no official way to test thickness short of popping the lid. This is frowned on in paint stores. There is an experimental method: I have heard that thicker paint is heavier in the can. Try weighing a cross-section of paint cans to find the paint-thickness you desire. I have not personally tested this method. It would be embarrassing to drag a bunch of paint cans to the nail scales, just to find out that they all weigh the same.
CHEAP VS EXPENSIVE
No contest. Cheap is always better, unless you are talking tools, cars, or women (and in cars and women this point is debatable).
Since paint can sit around for years and you have plenty to do before painting begins, there will be plenty of time to find paint on sale. There is no best place to buy paint. You will find that department stores will frequently have the same brand names as the hardware stores and lumber warehouses.