In old times (maybe before 16c.), the method of refining copper was not good enough to make it pure. So such old coppers include a small amount of other metals. Then they have a little different colours from pure copper. We call such coppers "YAMAGANE" to distinguish them from pure copper.
Yamagane is not a meaning of one kind of metal, but meaning of un-pure copper in old days. So it includes various kinds of coppers naturally. Usually they have a little deeper colour than pure copper's.
And later (early modern age), metal workers made such classical looking alloys by their will. It is also called Yamagane.

  
Above items are made of yamagane. And they have a little different colours from each other.
An old habaki before 15c. maybe.
A tsuba for naginata before 16c.
A pair of menuki in 16c.
 

This tsuba is also made of yamagane, but polished up recently.