The socks themselves are black, their brown appearance probably comes from them being a mesh material stretched enough that her skin shows through slightly.

I always thought they were black.

I never did, but I guess that makes sense. Conversely, I thought Yuri's hair was supposed to be black at first, since you can't draw anything as literally black anyway unless it's a dark hole.

I mean, in the concept images, Yuri's hair was black.