So far as gloss goes, keep in mind:
1. All other factors being equal, the flatter the paint the better it will hide an underlying colour.
2. All other factors being equal, the glossier the paint the easier it will be to clean.
And, so far as latex paints go, keep in mind that all latex paints are not equal. When you pay less for a latex paint, you're likely to get one made with a polyvinyl acetate binder. Polyvinyl acetate is a plastic which you probably have known up until now as "white wood glue" or "carpenter's glue". Polyvinyl acetate works fine in a dry room, but just in the same way as white wood glue re-emulsifies if it gets wet, polyvinyl acetate based latex paints lose their hardness and adhesion under wet conditions. So, these paints will often be found cracking and peeling on bathroom walls and ceilings, and this is often misdiagnosed as being due to insufficient prep work prior to painting rather than being properly diagnosed as using an inappropriate kind of latex paint in a wet area.) Also, adhesion is a hard thing to engineer out of the polyvinyl acetate molecule, so these paints will remain slightly sticky even when they're completely dry. So, if you're painting doors and door frames or windows and window frames, it's better to pay more for a better quality paint. Otherwise you'll continually have doors and windows "sticking" closed.
Better quality latex paints use a plastic resin made from polymethyl methacrylate, which you probably know better as "Plexiglas". This is a harder plastic than polyvinyl acetate, so better quality latex paints lose less of their gloss when you scrub them hard to remove a stubborn mark. And, these paints stick better to damp substrates, are much more UV, acid and alkali resistant and have much better resistance to wet conditions so they perform much better in a bathroom.
The wording "vinyl acrylic" and "100% Acrylic" is used in the paint industry to differentiate between polyvinyl acetate binders and polymethyl methacrylate binders, respectively. So, if you see a can of paint with "100% Acrylic" emblazoned all over the label, it simply means that the binder resin is made of Plexiglas rather than white wood glue. However, keep in mind that there are many different kinds of "100% Acrylic" paint resins, and they all have different properties and prices. So, saying a paint is "100% Acrylic" is kinda like saying that a package of meat is 100% cow. You could be buying sirloin steak, or you could be buying dog food. The trick is to buy a paint INTENDED for the purpose you want to use it for. Use a paint meant for bathrooms in a bathroom. Use a latex paint intended for masonary on brick or block walls. Use a latex primer intended for fresh concrete on fresh concrete. It's only then that you can be confident that the resin you're getting was chosen to provide the best service in that kind of application. The exception to this is "ceiling paints" which are just low priced wall paints, and "latex floor paints" which are simply too soft to provide good service on a working surface like a floor. I use latex wall paints on my ceilings.
You don't need to know the rest:
The reason why flatter paints hide better is that they use something called "extender pigments" to lower the gloss of paints. Were it not for extender pigments, all paints would dry to a high gloss. The flatter the paint, the more extender pigments in the paint, and the more coarsely ground those extender pigments will be.
Extender pigments are either white or clear rocks that are just a little too small to see with the naked eye. A "micron" is a millionth of a meter, or 1/1000 of a millimeter. A human hair (from a caucasian) is about 100 microns in diameter. The smallest thing that can be seen with the naked eye is about 20 to 30 microns in diameter. Extender pigments are typically 5 to 20 microns in average diameter, and a red blood cell is about 5 microns in diameter. So, extender pigments are just a little too small to see individually, but we see them collectively in their ability to make the surface of paint dry rough instead of smooth.
Extender pigments increase hide by reflecting and refracting light. Imagine you have a stack of 40microscope slides 2 inches thick and a glass block 2 inches thick. The glass block would be almost perfectly transparent but the microscope slides would be almost completely opaque. That's because every microscopically thin glass/air interface in the stack of slides will both reflect and refract light, whereas the glass block won't have those interfaces. Typically, 94 percent of the incident light goes through the glass, and 6 percent is reflected back. So, the percent of incident light that would make it through a stack of 40 slides would be (0.94)^40, and Windows Calculator tells me that percentage is
only 8.4 percent. And, assuming ALL of that 8.4 percent is reflected off the substrate back up through the stack, only 8.4 percent of the 8.4 percent would make it back out to your eye. That's only 7/10ths of one percent of the incident light that originally hit the stack on it's way in.
Consequently, so very little of the incident light makes it all the way through the stack and back out again that it's not enough for us to see. Since we can't see the light reflected off whatever is behind/under the stack of glass slides, that stack is "opaque" to our eyes even though each slide is transparent.
Clouds, snowbanks and the head on a beer are all opaque for exactly the same reason, even though everything inside all of these things is essentially clear and colourless. It's the scrambling of light by reflection and refraction that makes these things opaque.
Extender pigments (whether they're white in colour like titanium dioxide, zinc oxide, chaulk (which is calcium carbonate) or talc (which is magnesium silicate)) or clear and colourless (like pulverized silica sand (which is quartz)), all scatter light the same way as the water droplets in a cloud, so they make the paint film more opaque, and that means it hides an underlying colour better.
In general, I try to stick with satin paints for walls and velvet (one level flatter for ceilings). Satin provides a good compromize between the easy cleaning of a high gloss paint and the better hide of a flat paint. Ceilings don't need to be cleaned as much or as often, and so I opt for the better hide there. And, of course, a flatter paint will hide imperfections in the ceiling drywalling as glitches there can stand out like a sore thumb. That's because, unlike walls, ceilings are often illuminated by ceiling mounted light fixtures or by windows that come within a few inches of the ceiling. Those both cast light on the ceiling at a sharp angle, exagerating imperfections in a drywall or plaster ceiling. Painting the ceiling with a flatter paint helps hide those imperfections.
You can learn more than you need or want to know about latex paints at the Paint Quality Institute's web site at:
The Paint Quality Institute was established by the Rohm & Haas Company, which is the largest manufacturer of polymethyl methacrylate plastic in North America. Polymethyl methacrylate sheet plastic is called "Plexiglas" if it's manufactured by the Rohm & Haas Company, "Lucite" if it's manufactured by DuPont and "Perspex" if it's manufactured by Imperial Chemical Industries Limited (ICI Ltd.) of Britain. Rohm & Haas also produces polymethyl methacrylate plastic in the form of tiny particles (called "resins") which are far too small to see (about 1/10 of a micron in diameter), and these are used to make everything from latex paints, acrylic floor finishes, acrylic grout sealers and acrylic nail polish. The Paint Quality Institute was established to inform and educate home owners, architects and painting professionals about the benefits of using top quality latex paints. Since establishing the Paint Quality Institute, the Rohm & Haas Company has become a subsidiary of Dow Chemical.
The Bayer Company (the same company that makes Aspirin) is a major producer of the alkyd resins used in modern oil based paints, and was the first to patent the alkyd based "polyurethane" we still use today as "varnish" and "hardwood floor finish", back in 1956. Prior to that, varnish and hardwood floor finish were two completely different products as varnish was made from drying oils like linseed oil or Tung oil and hardwood floor "wax" was made from Carnauba Wax made from the nuts of the Carnauba Palm tree of Brazil. For over 50 years, alkyd based "polyurethane" has replaced both varnish and Carnauba Wax as the clear coat of choice over hardwood. Bayer markets it's alkyd resins under the trade name "Alkydal" through it's subsidiary "Viverso". Unfortunately, neither Viverso nor Bayer maintain an educational web site similar to that of the Paint Quality Institute to educate and inform people about oil based coatings.
Nowadays, real varnish is only used on wooden musical instruments like violins. Carnauba Wax is still used to make car waxes.