I know it's kinda late for this, but I think the color may be due to either cadmium plating, or a yellow zinc chromate-conversion treatment. The streaks were probably cause by turbulence in the plating tank (I believe the plating solution is kept vigorously agitated during the process) and this caused the solution to flow through the holes in the steel plate, which would have been punched prior to plating.
I think it's highly unlikely that the coloration was caused by the heat staking process that formed the "rivet" heads. This would have been a very quick operation, as the plastic melts very quickly -- probably too short a duration to so thoroughly discolor the steel and/or whatever anti-rust treatment that had already been applied. I envision the assembly being clamped into a holding frame of some sort while a curved heating platen was pressed down upon the protruding tenons (i.e., rivet shanks) to flatten them into round heads.
The heated-platen pressing cycle would have been quite short, probably only a second or two, and the platen would not have actually touched the steel. There would have been some radiated heat from the platen, but the short cycle would have limited its ability to cause discoloration due to the thermal "inertia" of the thick steel plate. I believe that to use heat treatment to produce a deliberate coloration on a metal object would require more than a second or two of exposure to mild heating. But then, I'm no metallurgist so I could be wrong.
