It certainly looks like it could be a border. I think this might be a good opportunity to use my darker gray as the neutral.
I had an exciting moment last night, as I put my first pieces (top left corner) of IW together. The pieces went together beautifully, so I slapped the section back up on the wall and took a photo . .. and saw that I had the red slash going the wrong way. So I took it all apart and re-did it. I hope it's the first and last time I make that mistake!