They aren't in color, rather they are just a black and white picture like that seen on a 1960 s television. Believe it or not, it s all just a matter of density. In our bodies different tissues and substances have different densities. Using that as a starting point let s put the radiographic picture of an animal on the film. Bone and metal have the greatest density of anything found in or on the body and they appear pure white on the x-ray film.
Air is the least dense and it appears black.
Soft tissues like skin, intestines, muscle, etc. are mostly water and they are also middle tones. The various soft tissues are outlined or differentiated on the x-ray by their relative percent composition of water, fat or air. After viewing a few thousand x-rays, you can easily pick out the various organs and other parts of the body on most films. You know where things should be and their relative density compared to surrounding structures. The problem with x-rays is that if something that shouldn't be there is the same density as everything around it, you'll never know it s there.