Those are known as "two tones" and a sure sign of being original Colt... they command higher prices than the fully blued ones.
You got a great deal... prices, as well as popularity, on these guns have gone nowhere but up since I bought mine in 2008. I put those wood checkered aftermarket grip panels on to preserve the original black hard rubber ones that came with it that are in excellent shape.
You could almost say that you bought two magazines and got a free gun with them.
If you haven't yet looked at
Colt Automatic Pistol and Revolver Information - 18 years of Service to Colt Firearms Collectors... do it, there is a wealth of information.