Here's my unsolicited 2 cents:
If you DO add sashing in whatever color you choose, may I suggest taking one block from the center column and undoing the 2 halves. Then use a half block at the top, then the 3 whole blocks, then the other half block at the bottom, with the sashing in between.
The measurements will work out exactly the same, it's not really any more work, and it will give the whole top just a bit more interest than the same-old same-old 3x4 layout.
Oh, and the color for the sashing = not yellow. ;-)