The actual patination process is the same all over but the various different starting conditions of the metal have created slightly different ranges of colour.
The very ridge of the spine was first TIG welded together and then capped with stick to build up a uniform section. Near the plates had a more or less random texture applied with stick. This worked by playing with travel speed to get an irregular weld bead which was still reasonably smooth with decent penetration.
Then the whole thing was skimmed over with a flap disk and allowed to rust (with the assistance of an acid etch)