Yours is nickel plated copper not bare copper.

Nickel plating wont tarnish like bare copper, and it looks better. Those are really the only two reasons I can think of. Someone else may be able to give a more technical answer as to why.

I assume it's for looks. The bare copper would turn green over time, so most heatpipes are coated in nickel. It doesn't affect the heat transfer very much if at all and keeps it looking good for a longer time.

that looks really good, Scarlet....Now I want it..hahah
 

It does take years for copper to start to oxidize. Especially in a low humidity environment. The Heatsink fins are aluminum and as such don't dissipate as much heat as pure copper might. Having the copper color below the fins might cause the white LEDs to look off color due to reflection. But most of the Heatsinks I've owned have used copper hestpipes that get plated as they leave the main contact point and merge with the fins.

Yea its not aluminium its definitely Nickel Plating. On another note I have never seen copper pipes or tubes change colour in a PC. Bonus note: 
For aluminium heat sinks, I recommend you buy them from an aluminium extrusion wholesaler as they will work out at a fraction of the price. You can purchase a full 6m length of a heat sink extrusion for the price of 2 commercial Heat Sinks. You can then cut them to any size you wish and use them where ever you want. 
Here is an example of a retailer where I stay, look for something similar:
 
 

Hoggle
I would wonder if the nickel is also inside the tube ...

No.