Will you share a link to which fabric? I see some WOW and some muslin, but don't see a solid.

it is called cobblestone.   https://www.connectingthreads.com/cobblestone-108-in-wide-backing/p/93981

I've also gotten faded chambray several times and I like it but it does ravel more than normal. (made LOTS of t-shirt quilts using that was sashing).   Dappled is very nice as is Tiny Dots & Mosaic dance is very silky.  The ones I got recently that I wasn't pleased with was Word Play and Daydream.  

They put their wide backing on sale 25% off about one a month or every other month.  During the Thanksgiving holiday they were having daily sales, at one point it was 30% off and then later everything was 25% off.  You just have to watch their emails.

When ordering, if my theory is correct, look for Connecting threads instead of another manufacturer under the fabric name.

Thank you!

The center is now sewn and I tried your idea about the border, but I'm not sure how I feel about it.  Also a little concerned about how to measure and get things to line up.  I think it's going to have to "age" a while.  I will probably just be bringing the unfinished top to the retreat for my Sharon honorarium.

That is not the idea I had but it does look great. I think bringing those colors out to the border really adds to the composition. You could try making them thinner which might make it easier.

LOVE the diagonal lines of color.  Just superb.

Regarding the border: how about angling the ends of the color strips to mimic the diagonal lines in the interior?  If you really wanted to be finicky about it (what? you??), you could leave a black spacer (also on the diagonal) between each angled color strip.

I was planning on angling, this was just to get an idea of what it would look like.  But I can't figure out how to do the math to make things line up properly.

Don't worry about the math.  In fact, who cares about the math?  Forgetaboutit!  Make a paper template for a mock-up.  

Once you have that inner black border sewed on, put a strip of paper alongside.  Just draw the angle on the paper for the finished template.  Remember to add your seam allowance!!

Then, before you cut your actual fabric, do a mock-up or two in practice fabric to make sure that the angles really are correct.

I love it with the colors like that but it looks challenging!

for some reason I was thinking the lines weren't consistent, like radiating from the center.  But they are parallel.  not difficult.  I'm  almost done piecing the border, lots of bias, which is good and bad.  Gives me some wiggle room, but it does wiggle a lot.  I'm adding some stay stitching and lots of starch.  Now I need to decide just how wide I want the black spacer.