When I segmented 3D MRI and CT scan images before I used the contrast borders for help a lot. There were some algorithms for finding edges that you could tune by setting search radiuses and thresholds. There was also an option of growing an area by a certain amount of pixels outward, and then threshholding the result back down to only the brighter parts, that kind of thing. You had to be a little clever about how you'd combine it. And ultimately, sometimes I just had to add and subtract a few points manually.
Segmenting is more assigning areas to distinct objects (separating bones from the rest in my case), but you could totally use it as a basis for coloring, so I assume the process is similar here.
These are manufactured differently from most of the stuff you'd be looking at.
Rather than milling and grinding, the needles are made from a sheet of stainless that's rolled and welded, then drawn down to whatever size it needs to be, basically stretching the material out. Kinda like when you make a snake with silly puddy and pull it apart.
Then the points are ground in. Gives you a ridiculously smooth finish.