Why is one 340 connecting rod bronze color?

RAMM said

Wrong. Copper is way soft and would not hold up to the beating of a rod journal or piston pin can dish out. Copper is the material but it was done merely for corrosion resistance. I have seen engines with all of the rods copper coloured. The bean counters surely put a stop to it. J.Rob

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It will hold up just fine in either end of the rod, but seems to be done mostly to tighten the small end. Since the pin is a press fit in there, the copper is effectively a shim and will work just fine in such an application.

Copper corrodes just as bad if not worse than cast iron, so it wouldn't have done diddly for 'corrosion resistance'.