Are there any online tutorial for painting dinosaur models? I can't seem to find any tutorials on how to paint the skin.
My best suggestion is to learn how to use an airbrush - it you don't already know how. Most of the best painting is done using an airbrush. You can also go with oil paints, applied with various artist's brushes. After assembly, wash the model with warm water and Dawn detergent to remove any mold separator, and oils from your own hands ... wear vinyl gloves while washing the model. Paper towel it dry, then let it fully air dry a couple of days. The first thing is to lightly coat the model with a can of gray spray primer. allow this to thoroughly dry, then proceed to paint your model. Generally paint light colors first, adding darker shades and markings next. After painting you can seal it with a matte acrylic spray. Following this, you can 'weather' the animal with a wash of dark brown or black Acrylic paint to bring out some of the scale detail. It did this on the belly of the beast. A final coating of matte spray finishes the project.
As for how
to paint them, anyone's guess is a good one! There is no real knowledge on what they looked like color-wise, but assumptions can be made from modern animals. Predator vs Herbivores in the way of coloration, and possible body color camouflage. Browns, grays, and various shades of muted greens would be good choices for general body colors. Brighter reds, oranges, and even blues can be used for certain parts of male Dinosaurs ... such as head the cranial crest of the Parasaurolophus, the head shield of Triceratops, among others. These are reasons why a tutorial would be of little help.
Best of luck to you in your modeling!