As far as the real color to use, it does not much matter. Choose the color you like best in all the squares. Then, use a "dark" value of that color. If you like green, use a dark green. It does not have to be too dark, but it needs to be darker substantially than your darkest green in the blocks. If you like Red, use a dark red. The sashing in a dark value will make the blocks "pop out"...If the sashing is not dark, your blocks will merge with the sashing and you will not be able to see them. If you choose a white sashing, that will work, but any block that is almost white will get lost also. Keep the print tiny in the sashing since the prints are tiny in the blocks. This would look great with a different color squares at the intersections of the sashing. It would even be great with small star blocks in the sashing intersections...
It will be lovely in any colors you choose. Please post a new picture when done.