I agree with you, vcquan. Deciding about colors and widths of sashes and borders can be really frustrating. And everyone has great suggestions. I often lay the fabric I'm thinking about using for sashing under the blocks to audition the color and/or width. It helps me 'see' what the final project will look like.
Personally, I like the idea of a dark grey or black for sashing. But, if you use black, I would take out the black blocks. The sashing won't show up if it's the same color as a block and that particular block will stand out like a sore thumb.
I would suggest that you not use white in your outermost border. It's a great background on the blocks but it's not different enough to pop out and 'frame' your quilt picture. And, there's always that, 'white gets dirty faster' thing, too. Again, I'd use black and the yellow that you've picked. I like to use 3 colors in my borders.... with graduating sizes starting skinny and going bigger..... like, black (1"), then maybe the white because at that spot, it's not touching your blocks (2 1/2") and the bright print or yellow (3 1/2"). You'd have 7" of border. That sounds like what you were originally thinking.
You just have to show us what you come up with when you decide. Decisions are so difficult.