My first thought would be to do a narrow framing border of the lighter green. Then either the darker green or the orange to build up the border to be the size of the cornerstones. Either would look good, it just depends on which one you prefer. If you do the narrow light green framing, you could even do the 'regular' border in white and it would morph into the triangles in the corners. I love this site because you always see quilts that have colors you would not normally put together and then you see then and realize how nice they look! Good job!