Though this tidbit of knowledge has been widely distributed as part of an internet "Did You Know?" list, at no time in Coca-Cola's history has that beverage been green.
The original formula called for caramel to give Coca-Cola its rich brown color, and although the recipe has undergone some changes through the years, none of them affected the ultimate color of the product.
Brown also hides impurities in any given batch, something the backroom chemist who invented Coca-Cola in 1886 kept well in mind as he proceeded with his formulation. These days syrup producers and bottlers have no impurities to hide, but back in the "three copper kettles in somebody's basement" days, covering up what might have inadvertently dropped into the mix was a concern, and brown hid indiscretions remarkably well.
Coke has at times been bottled in green glass bottles, which perhaps explains the popularity of this particular rumor.
Coca-Cola's own website notes of this claim that:
This is indeed just a rumor. Although the famous contour bottle is green, Coca-Cola has always been brown in color, since its start in 1886.