Now that is a really lovely block. The sashing fabrics look too bright to me. Have you considered pale cream or a deeper green, something a little darker than your darkest green so far? Or perhaps a green that's a bit darker than that darkest green but not as light as the other greens. One thing that stands out to me is that the darkest green is noticeably darker than the rest of the fabrics. There's nothing wrong with that, it can add a lovely effect, but it's something to be aware of.
Something I find very useful for this sort of job is Joen Wolfrom's 3-in-1 Color Wheel. It helps you understand colour theory, get the right colour, and the right amount of black/white/grey in it.