To make redemption and sorting easier, companies differentiated denominations and classes by various colors of border designs. Some companies issued as many as eight different colors for stocks of the same issue. One has to wonder, though, whether those companies were redeeming and re-issuing so many certificates so frequently that multiple denominations were absolutely necessary.

There seems to be no reliable relationship between the color of stock certificates and denomination among the surviving population of collectible certificates.

Remember, the colors of stock certificates were primarily used to help companies and clearing houses differentiate betweeen denominations quickly. The colors themselves might have varied substantially, as long as they aided in sorting. Read more about colors at How variable are certificate colors? Also, if you want to report new colors, please read Reporting new certificate colors.