Dark orange probably. Just kidding, I did some quick research and found that the original formula called for caramel to give Coca-Cola its rich brown color. The recipe has gone through some changes over the years, but if it didn't have any dyes or coloring in it, then the color would just be a bit lighter then it is now.
Without any coloring its main liquid is water so it would be clear. Corn syrup used to sweeten it might give it a very faint amber color. The other main ingredient, carbon dioxide or carnbonation is clear.