is the dark brown a different color than the choco of the t? if so, i'd just use the color of the t for the brown which is kind of how it looks in the preview already.
It's a tad bit darker brown. I didn't think it was defined enough to tell what it was when it was the same color.
The color doesn't really matter that much on the template though, if people still like the design, but mention that they like a different color, Threadless will probably change it to fit what people want.
2 color design- Dark brown and light blue on a chocolate colored tee.