I have a grand marnier cream cheese “mousse like” filling for my pumpkin roll, and I use a star (large open star) to pipe the outside with it. The ridges resemble bark and you can swirl it on the knot in the middle if you use one. As for coloring it, I use it plain, but an airbrush will probably give you the shading you want.
For traditional buche de noel, I use either ganache or a dark chocolate buttercream (using Callebaut callets). The buttercream allows for natural shading when you go to make it look like the bark of a tree after applying it.