There are a few basic things everyone should know about paint:
1. Don't use your standard wall paint in the bathroom. If you do, you're likely to get paint cracking and peeling on the ceiling above the shower or tub and high up on the wall where the highest humidity is and condensation forms. Use a paint specifically meant for bathrooms like Zinsser's PermaWhite Bathroom paint.
The reason why is that there are three different kinds of resins used to make latex paint:
1. acrylic (typically called "100% Acrylic" in paintspeak)
2. vinyl acrylic
3. styrenated acrylic.
Not only will Bathroom paints use acrylic resins, which have the best moisture resistance, but they will use the most moisture resistant 100% acrylic resins available. "Budget Priced" paints typically use Vinyl acrylic resins which have poor moisture resistance, and the result is that these paints will soften and peel when they get wet or under humid conditions. The resultant peeling paint is often misdiagnosed as poor surface prep before painting. Also, bathroom paints will have much more mildewcide in them to prevent mildew growing on the paint.
2. Two good tests of paint quality are both hardness and hide. The biggest hunk of the cost of a gallon of paint is the cost for the resin "binder", which is the plastic that forms the film. The higher quality the latex binder in the paint, the harder the film it will dry to. That's cuz the harder the paint film, the less easily it will be damaged. So, If something is rubbing against the wall, like a bed head board, it leaves less of a mark, also you get fewer "scuffs" that can't be cleaned off and need to be painted over, and cleaning marks off walls won't dull the paint's gloss nearly as much. Cleaning a mark off a wall only to have the resulting dull area stand out when light from the window reflects off the wall is a kick in the rear cuz it looks way worse the original mark.
3. Since the binder in paints will dry to a clear or transluscent film, hide in paints comes entirely from the pigments in the paint. In the case of white, or off-white paints, the ability of the paint to hide an underlying colour depends largely on the amount and type of the white pigment titanium dioxide there is in it. Within limits, the greater the amount of titanium dioxide of the right particle size, the better the paint will hide. Since titanium dioxide is the most expensive of the white pigments, higher quality paint will normally contain more titanium dioxide, will hide better and will cost more. Lower quality paints will use other white pigments like zinc oxide, lithopone, calcium carbonate (aka:chalk) and magnesium silicate (aka: talc). These white pigments don't hide as well, but they don't cost as much either, so they're often used in less expensive paints.
Titanium Dioxide in Coatings - White Pigment - Opacifying Center - TiO2 - SpecialChem for coatings and inks
4. All things being equal, a paint with higher gloss will be easier to clean but won't hide an underlying colour as well. A paint with lower gloss will be harder to clean, but will hide an underlying colour better. The reason for the hide to depend on the gloss is because the lower the gloss of the paint, the more "extender pigment" there is in it. Extender pigments are huge rocks almost large enough to see with the naked eye. It's by adding extender pigments to paints that you make paints dry to flatter glosses. Without extender pigments, all paints would dry to a high gloss. Those extender pigments improve hide by scattering light. Think of shining a flashlight through an aquarium to see a coloured wall on the other side of the aquarium. You could easily see the colour of the wall. Now, pour glass marbles into the aquarium instead of water. If you now shine the flashlight through the aquarium, the colour will be very much fainter because much less of the light from the flashlight could make it all the way through the marbles, reflect off the coloured wall and make it all the way back through the marbles to your eye again. Most of the light you'd see would come into the aquarium through the sides, top and bottom and reflect and refract until it eventually hit your eye, and that light would not be the colour of the wall on the other side of the aquarium.
(you can prove this with a simple experiment. Try looking through a 2 inch high stack of microscope slides. You can look through a slab of glass 2 inches thick, but not a stack of glass microscope slides 2 inches high. The reason why is that 6% of the incident is reflected back to your eye at the surface of each microscope slide, so that the stack of glass slides is completely opaque, while the 2 inch thick slab of glass is almost completely transparent.
