Looking at the pictures it seems like a different metal was used for the silver parts, so also a different color was intended. Goto masters often worked with Shakudo, Silver and Gold in one work.
The small marks are no signatures unfortunately. Goto signed their pieces quite clearly. However, there are some secret signatures on Goto pieces in form of small dots often on the side of the pieces. These were made as kind of a secret sign by a later master, that the piece is genuine and from an earlier master, if I remember correctly. You can find some more info about how to identify goto pieces under tsuba.info/goto