Ichthyornis has the skeleton of your average bird, almost. There are a number of differences of course, the most obvious being the beak of the bird. Small conical teeth lined the upper and lower beak in life and were well suited not only to grasping fish, though not as well suited to this task as other piscivorous teeth, but also to prying mollusks from their shells, such as clams, and most likely they were even strong enough to break open some smaller shells, such as those found on snails. If it were alive now I have no doubt that Ichthyornis would also prowl ocean side parking lots for onion rings and french fries, but it made due with clams and fish and other small ocean prey items and was quite happy for them I am sure. Those teeth have also served as a source of controversy since they were discovered. Prior to Ichthyornis no bird or near bird had been discovered with teeth, Archeopteryx skulls with teeth were not found until the 1880's, and this skull which could be used to support the theory of evolution that was becoming ever more popular was feared by some. Marsh, who possessed the finest examples of these skulls, was urged to keep the teeth secret and some theorists conjectured well into the 1960's that Marsh had fabricated the fossilized skulls and teeth of Ichthyornis.
The truth is that Marsh did not even recognize that he had a bird at first. The reptilian jaws and concave vertebrae suggested a fish or marine reptile and Marsh fully believed, until 1873 that that is what he had brought home to Yale from central Kansas, a reptilian jawed fish or a marine reptile with a fish's backbone. During that year, 1873, Marsh and his preparers chipped tediously away at the rock until they realized that the jaws and vertebrae were attached to a very birdlike skull and that the rest of the body was an almost completely modern bird's body. This gap spanning species was an astounding find which linked two worlds in a new and exciting way (in all fairness he knew there was a bird body but he thought the jaws came from a fish and were not initially connected to the bird body). The discovery that the skull, teeth, and body were all from one single animal and not a bird and a fish prompted Marsh to create a new order of animals that he called Odontornithes, or toothed birds. This order does not still exist but held not only Ichthyornis but also Hesperornis. Today they are separated, but Marsh's clade, the Carinatae, still exists and Ichthyornis is a member of this clade (it is also an Ichthyornithiforme whereas Hesperornis now sits in the order Hesperornithiforme; classification is a science all to its self).
The most important thing to remember if you want to think of the living form of Ichthyornis, though, is that it was very much like a modern seagull in terms of its niche in the environment and food chain. Given this niche it is likely that the coloring of Ichthyornis was like that of a modern seagull as well since the general color scheme of beaches and the ocean has most likely not generally changed over the eons of time separating the two species. Gulls, however, are not actually related to Ichthyornis and therefore we could also come to the conclusion that these two genera may have had nothing in common in coloration. It's a big world and there are lots of possibilities that we will not fully unearth without a ton more evidence, though, it should be known that Ichthyornis is one of those Cretaceous birds which has had quite a wonderful number of samples come about and rear their heads out of the fossil beds of the inland seaways.