well, i wanted a brown for it as I'm not too wild about greys and those are the dominate shades. But I didn't want a dark brown. It isn't as "delicate" in appearance as the White Tortoise so I figured a medium brown would work well. After some dinking I ended up putting Diamine Sepia in it which is not a medium brown.

And I think it's an acquired taste. At first i didn't like it, the more I use it, the more i appreciate it. It's a golden brown, fairly light.
A better "match" to the pen is probably something like PR Copperburst which I think of as a medium brown with red highlights though i found it a dry writing ink. If you like gray ink though - that would work well also.