The most common adjective color associated with seagulls in the United States is "gray." This is due to the predominant coloration of the most common species found in the country, the Herring Gull (Larus argentatus). Herring Gulls are easily recognized by their gray plumage, which includes their backs, wings, and underparts. This gray coloration is a key characteristic that differentiates them from other types of seagulls, such as the Black-backed Gull (Larus marinus), which has a predominantly black back, or the Glaucous Gull (Larus hyperboreus), which is predominantly white with a gray back.

The gray coloration of the Herring Gull serves several ecological functions. Firstly, it provides camouflage against the overcast skies and the rocky coastal environments where these birds are often found. This helps them blend in with their surroundings, making it easier for them to hunt for food or avoid predators. Secondly, the gray coloration is a result of the pigments melanin and guanine, which are also found in other birds and animals. These pigments can reflect UV light, which may help to regulate the bird's body temperature in the harsh coastal environments.

Data from the United States Geological Survey (USGS) and the Audubon Society indicate that the Herring Gull is one of the most abundant and widespread seagull species in the country. Their gray coloration is a significant feature that contributes to their identification and ecological success. Additionally, studies have shown that the coloration of birds can have evolutionary implications, as it can affect their survival and reproductive success. The gray coloration of the Herring Gull is a prime example of how coloration can be an important adaptation for a species.