My next project is to take this wagon (which I didn't create) and make it look not so gray. It's not too hard to slap a color on it or make it shiny or dull. The tricky part is tweaking things to make it look more plastic or metallic or whatever. Then it gets harder when you stack multiple surface features onto one object like this wheel for example.
The wheel has a plastic center and a rough tire on the outside. However, the model itself is just a single object. So I had to take care of both material types in a single shot. I had to layer on different characteristics which is graphed below. Now I can understand the individual parts of the graph, but I'm not sure if I could recreate it without having a reference. It's not as intuitive as I would like but hopefully it'll get better over time.