[Kamikaze Chinchilla]

Low cut chucks xD

It's up to you. You can use something like pen and use the water tool on it later to blend it in, or you can try the airbrush. I would recommend doing the shading on a separate layer, using gray, set the layer on multiply, and all your colored work beneath that layer - so if you mess up, you can fix it.

Some people do all their work on the same layer, all in gray tone.