Human beings are naturally attracted to colorful things, whether it's a car, an item of clothing or an advertisement. Make sure that your beer poster is colorful, but design the poster in an organized way rather than just mixing bright colors together.
Make sure your colors accentuate the product you're advertising. For example, if your beer is an Oktoberfest brew that is typically sold during the fall or winter months, a nice seasonal scheme of colors like orange, red, and yellow would be appropriate. A beer designed for the spring or summer, on the other hand, would be better suited with lighter colors like blue and green.
If your beer is from a certain region, you may want to use culturally appropriate colors. For example, a beer from Louisiana might incorporate the colors of Mardi Gras: purple, green and gold.
For the standard, year-round beer, it's best to just stick with one color scheme that matches the taste and/or color of your beer. Bud Light and Coors Light are basic American beers, so they use red, white and blue on most of their advertisements. Guinness, on the other hand, is a darker beer from the colder region of Ireland, and it makes sense for their posters to incorporate colors like black, dark brown, tan, and green.
While your poster may appeal to the eyes with just a color scheme and the beer logo, it's important to incorporate a slogan or message to make sure your product will actually sell.
Some beers will adopt one slogan and stick with it, like Bud Light's "Drinkability" campaign or Miller Lite's "It's Miller Time" slogan. If this is your preferred method of marketing, it never hurts to use variations of the same slogan. If your slogan incorporates the word "smooth," for instance, you may come up with modified versions of your slogan by simply using the word "smooth" in differing situations.
Other beers may prefer to come up with a message. There are multiple ways of pushing your product through one or two sentences on a beer poster. The most common method is humor. A clever phrase or pun may work wonders for attracting attention from bar-goers, and humor is typically the most effective way of advertising (especially to the beer-drinking demographic).
Some companies may prefer to use another technique: nostalgia. Many beer posters will incorporate old-fashioned images, such as a horse-drawn carriage or a jukebox, to invoke simple pleasures from simpler times and accompany the image with a sentence or two about "the good ol' days." Because beer is a centuries-old tradition, it never hurts to use a throwback poster scheme.
No matter what color scheme you go with or whether you choose to go with familiarity, humor or nostalgia to sell your product, you need a logo. It really doesn't matter what the logo looks like--Samuel Adams has a picture of its historical namesake in its logo, but Miller Lite simply changed the spelling of "light" and surrounded the letters with colorful circles. Whatever road you take, a logo is necessary so that people will be able to identify your product no matter how many beer posters you put up.