They are forged, I expect they first upset the end (make it larger and or with more metal) then finish forge the flange to shape, if not done all at one wack.
The color is on top of the finished machined part, so the heat was applied after machining. I see no evidence of spin welding a flange to a shaft, and even if so it's likely the flange was finish machined after.

I still suspect the heat/color is a result of heat treating and or tempering the shaft at the very end of the whole manufacturing process.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-