Thank you all so much for your compliments, suggestions, and encouragement! I finished the top and I must say, I am very very happy with it! This is my first-ever-completed quilt top.
I decided to use a royal blue mottled fabric from Moda for the sashing. It's the color I had in mind all along, so when I found it at my LQS, I knew it was meant to be. Then, I wanted to add something at the corners of each block, and found this adorable PP block called Geometry on Quilters Cache. It's a 4" block, but I re-sized it to 3" and used all the colors in the quilt. I think it ties everything together nicely. Finally, I wanted to cap the whole thing off with a little more of the rainbow colors, so I pieced a strip of each color together on the diagonal and bordered the whole quilt. I love it!!!
I never expected anything I made myself to turn out so well. There was A LOT of remeasuring and (re-cutting ) as I worked my way through the mathematical calculations, since I didn't have a pattern or instructions to work with. But, I thoroughly enjoyed the process. Thanks again!