Amcraftma: Thanks for taking the time to read it.
You can learn more than you need or want to know about latex paints at:
Painting information and resources for home interiors and exteriors - Paint Quality Institute
That's the web site of the Paint Quality Institute, which is an organization that was established by the Rohm & Haas Company, a company few people have ever heard of. The Rohm & Haas Company was founded by Dr. Otto Rohm and Mr. Otto Haas back in the 1900's to sell products to the leather tanning industry. However, Dr. Rohm had done his PhD thesis in the polymerization of acrylates, and remained interested in that subject throughout his life. He was the first one to both find an economic way to make the feed stock for acrylic plastic, and the first one to find a way to cast acrylic plastic into sheets. In 1933, Dr. Otto Rohm patented and marketed cast acrylic plastic sheets under the trade name "Plexiglas".
The Chemistry Chronicles - The Long Road to "Organic Glass"
The Rohm & Haas Company made Plexiglas out of the plastic known as "polymethyl methacrylate", or PMMA. It was used to make airplane windows, canopies and bottom gunner turrets on airplanes during WWII. However, that same plastic can be made into a wide variety of different shapes. If you make polymethyl methacrylate into tiny blobs, each about 0.1 microns in diameter (or about 1/50 of the diameter of a red blood cell) then it is called a "polymer colloid" and can be used to make the "binder" in good quality latex paints, acrylic floor "waxes", acrylic grout/masonary sealers and acrylic nail polishes for women.
The Rohm & Haas Company was the largest producer of polymethyl methacrylate in both sheets and as "polymer colloids" (or "resins" for paints, floor finishes, etc.) in North America until the company was purchased by Dow Chemical in 2009.
A good quality latex paint is in fact a "slurry", which is what you call solids suspended in a liquid. The solids in this case are the polymer colloids, coloured pigments and clear, transluscent or white "extender pigments" all suspended in a solution of water and a low volatility water soluble solvent called a "coalescing agent" or "coalescing solvent".
Polymer colloids are tiny hard "blobs" of plastic that would be transparent and colourless if they were large enough to see. But, because they are so small, a suspension of polymer colloids in water would be milky white in colour. That's because the tiny clear polymer colloids reflect and refract light the same way that water droplets in a clowd do, and your eye sees the resulting mixture of every different colours of light as the colour "white".
When you apply a latex paint to a wall, the first thing that happens is that the water evaporates. The result is that those polymer colloids find themselves immersed in that coalescing solvent at an ever increasing concentration. The coalescing solvent dissolves (kinda) the polymer colloids making them very soft and mushy. The same forces of capillary pressure and surface tension that make small water droplets coalesce to form large rain drops in clowds then come into play and cause each soft mushy polymer colloid to stick to and pull on it's neighbors. The result is a continuous film of soft mushy plastic with the coloured and extender pigments suspended inside it very much like raisins in raisin bread.
As the coalescing solvent subsequently evaporates from the paint film, filling the room with that "freshly painted smell", the plastic film hardens up again to the original hardness of the polymer colloids (when they were still in the paint can), and what you have is a hard film of clear plastic on your wall with tiny coloured pigments and clear, transluscent or white pigments suspended inside that film to give it colour, opacity and (depending on the extender pigments) it's level of gloss. Lots of large extender pigments results in low gloss, and without extender pigments, all paints would dry to a high gloss.
If the paint you applied to your wall was of good quality, that plastic film will be made of polymethyl methacrylate, the same plastic that Plexiglas is made of. (!) (most people don't know that)
And, if you understood that film formation process, you realize that it results in the elimination of all the solid/liquid interfaces that previously reflected and refracted light at the surface of each polymer colloid. So, the darkening of latex paint as it dries is entirely due to the elimination of those solid/liquid interfaces as the polymer colloids coalesce into a solid plastic film.
Typically, however, people refer to the kinds of polymer colloids used to make latex paints as "binder resins". If you hear the word "binder" or "resin", it refers to the plastic blobs in the wet paint that coalesce to form a solid plastic film that everything else is suspended inside. You have to consider the context to know whether they mean the polymer colloids in the wet latex paint or the solid continuous film of plastic in the dry latex paint. They say "paint binder resins" because polymer colloids made of PMMA have many other uses other than for making paints.
There ya go. Most people working in paint stores don't know why latex paints darken as they dry; they just know that they do. Now you know WHY they do, and can explain it to them.
None of this stuff is complicated. It's just that it's not taught anywhere, so there's a widespread lack of knowledge and understanding about this stuff. And, lack of understanding is fertile ground for misconceptions and misinformation to come into existance and spread like wildfire.