The tan material (glass-filled alkyd) was an upgrade from the black material (bakelite); it is less water-permeable and has higher dielectric strength. That is why Chrysler phased in the tan caps across all applications in the '64-'65 timeframe, on ~all engines. Not enough of an improvement to warrant any TSBs that I've ever seen, but enough to comport with "The policy of Chrysler Corporation is one of continual product improvement. Specifications are subject to change without notice".
Whether any given car got a black cap or a tan one was down to what was in stock/came to hand when a particular distributor was being built. You can follow the part number supersessions for any given type of cap through the years, and it goes black-black-black-black-tan-tan-tan. The next material change was in the mid-late '80s, when glass-filled thermoplastic caps began to replace the alkyd ones, for the same reasons (plus thermoplastic was cheaper).