Pretty blocks! Purple or green would look nice...I'd probably go with a mid-tone, not too dark. I think I'd go wider than the border on the one in the bottom row. You could also do one color for sashing, another for cornerstones. The post that has the alternating borders is nice too.
I'm a border lover...I usually have at least an inner and outer border on mine. I like how it frames it up...like matting and frame enhance a piece of art or photo.