If I were going to bring in another color, I would find a print that has that color mixed in with either the brown or the beige used in your blocks. Using that print for sashings and first border, you could then add a more saturated print of your color for a second border. Perhaps even a solid of your added color for cornerstones if you wanted.
I would go for the blue because I love blue with browns, but you could get away with so many different colors; red, orange, green. I would stay away from purple just because the purples and browns could tend to muddy each other.