We know a little bit about how dinosaurs probably looked: we’re learning that a lot of them had feathers, and even beginning to be able to figure out what patterns might have been on their plumage. But there’s a lot we don’t know; we may never know for sure.
In evolutionary biology, when you don’t know something about an extinct species—what kind of nest it built, or what sounds it made, or how many babies it had—you look at the species that evolved from it. You infer that it probably was similar to at least some of its descendent lineages. For dinosaurs, then, we would look at birds.
I remember, as a kid, thoughtfully coloring my dinosaur coloring books and paper-mache dinosaur scultures in shades of green and brown. I knew about camouflage, and I was sure that this earthy color palette must have been the one favored by these animals. Silly me.