For quite a few years now, the typical new car comes painted in a single color. But from the 1920s through the 1950s and even beyond, many cars came painted with two or more colors, two being the most common variety by far. They were called "two-tone paint jobs" back in those days.
The purpose of using more than one color to paint a car is purely aesthetic, even cosmetic. Color choices might make a car appear a little taller or shorter or even longer, depending upon where the colors were applied. Or features such as fenders or aspects of body shape might be highlighted.
Why did two-toning fall out of favor. I don't know for sure, but offer two reasons that make sense to me. One reason had to do with a reaction by American automobile stylists against the excesses of the late 1950s when tail fins sprouted and three-tone paint jobs appeared. In the 1960s car design became more restrained and single-tone paint helped reinforce the new seriousness.
Another reason, more appropriate to the 1970s and later, was related to the movement to improve efficiency and lower the cost of building cars. In the 1950s, for example, car makers offered many options that a buyer could select or reject -- radio, air conditioning, type of transmission, powered versus hand-crank windows, bumper guards, and many more including choice of paint from a list of currently available colors. The number of such options reached the point that the number of possible combinations became astronomically large. Potentially each car on the assembly line would differ in some way from all the others. This was abetted by the fact that many buyers ordered a car with the exact set of options they wanted rather than accepting a car available on the dealer's lot.
A practical result of all this customization was a decrease in quality because workers had to vary their tasks according to the whims on a car's options tag. And it was difficult to make sure that required parts were available when needed.
By the 1980s American manufacturers were following the Japanese practice of offering a limited selection of options packages. One result was that a buyer usually couldn't get his exact set of desired options when selecting a car. Another was that manufacturers could build vehicles more efficiently with better quality results.
By eliminating two-tone color schemes, car companies greatly reduced the number of combinations moving along assembly lines. Consider: Assume four accessory packages and ten available colors -- that's 40 combinations. Adding an unlimited selection of two colors from the list would result in hundreds of combinations.
That said, let's harken back to the days when multicolor cars were common and take a look:
These are examples of two-toning during its post- World War 2 heyday. The Buick has its top's color extending down a raised section of the hood. The Pontiac's top color does not cover the upper part of the doors, though Chryslers of that vintage had the top color extending down to a chrome strip mounted just below the bottoms of the side windows.
Here we have three-tone paint jobs. Actually, these were more limited than one might think; typically the colors were black, an off-white and a bright color of some kind. This prevented ugly color clashes that might make a company's cars look ridiculous and it simplified manufacturing. The Packard shown here is painted white along with two other colors. Most Packard Caribbeans of the 1955 and 1956 models years were like the Dodge, using black, a white and something else.
In the 1920s and early 30s two-tone applications were similar to that of the Studebaker shown here, fenders having a different color than the body -- though the top of the body often shared the accent color of the fenders..
Here are two Ruxton survivors from the 300 or so ever built. These have multi-tone paint jobs, the colors selected and patterns designed by the prolific, highly talented Joseph Urban. The car at the top is in the Tampa Bay Automobile Museum, and seems to sport at least four colors. The one below is in the Blackhawk Museum in the San Francisco Bay area and I count six colors on it.
Finally we have a Citroën with a color scheme by modernist designer Sonia Delaunay. The only photo I can find is in black and white, so I don't know how many colors were used -- at least four or five -- or what they were.