I think your next-to-last sentence says it all: It took you six months to find the right one. Do you really want to reject it now because of the color of the chairs?
Are they comfortable? That's really what counts, because you'll be spending an awful lot of time in them.
They refused to fix it, and you agreed to accept it as-is, so it does sound like this is a theoretical question now. If it turns out that they bother you more later, rather than less, you can either recover them or sell them and buy something else.