Lovely ojime collection strand!

I believe your coppery colored ojime is made from shibuichi, a copper/silver alloy. Shakudo is a gold/copper alloy which is normally blackened. In fact both shibuichi and shakudo were normally patinated to bring out a range of colors which depended on the exact composition of the alloy.

Here are two ojime from my collection - with shibuichi base on one, and I believe the black base on the other is shakudo, but not 100% sure.

The attached link has some info on Japanese metal alloys.