This is a big help. I love Gay's Idea. It will work for a quilt I am working on too.
Thanks for sharing.

What about adding a piping to the seam between center and border. That would break up your color line as you wanted, but not add inches that require more blocks.

This is why I love the Quilting Board! These are all workable solutions and I would never have thought of any of them.

Your quilt is beautiful and one of these great solutions will allow you to make the changes you want.

This was going to be my suggestion as well, after you check to make sure the outer border fits your centre.

It appears that your outer borders are made up of either FG and HST or entirely HST and those are the sections you have not sewn yet. I Would do a 1/4” black border on both sides of the border. That would mean you would only have to makeup 1/2” total. I would do as Kalamazoo suggested and sew those HST seams each a few threads narrower, enough to make up the 1/2”. bTW your quilt is beautiful.

You may be able to make enough bias tape to sew between the main section and the outer border without changing the dimensions of the outer border. I did this once to cover up a mistake that I made where a triangle did not match up with a block leaving 1/2 an inch of fabric showing where it should not. I covered those lines with 1/2 " bias tape.