You need to explain why you know it is brass and not copper. BTW, every Lincoln cent 1909 - 1981 contains 5% zinc (and sometimes some tin) with 95% copper, Technically, that is brass
If the date had been post-81, I would suspect he was referring to the zinc core Lincolns that are faced with bronze alloy rather than the pure copper layer. Ken potter especially has been promoting these mint errors. The bronze develops according to the mint from dissolved zinc blanks contaminating the copper plating solutions. Perhaps the OP is referring to a color difference similar to this occurrence, which wouldn't be possible before the copper/zinc cents starting in 1982.