Thanks for the tips - will look at some Hardie this weekend. I have a finish nailer and stainless nails - but I also have some trim head stainless left over. Not necessarily hardie-specific, so might need to look.
I also received two samples of the composite board from NewTechWood. Stuff is heavy - not sure how it compares to trex (mold) - and is real tempting... but I feel like it will look fake.
I'm also looking at reclaimed barn wood on Monday.
Some progress - the color is a lighter blue/grey/green but due to how bright the sky is, appears darker: