Quick suggestion - if you're going to do the wood accent, it needs to wrap the corner of the house, otherwise it looks like it was glued on over the actual siding.

Even though siding isn't structural, it should look structurally directed.

Since you're in a cool/wet area, if you want to make the home feel more welcoming, look at warmer colors, like stone or cream, instead of cool grays and whites. It will still look modern, but won't feel so depressing.

H Andrew - are you able to show us an all white house with the first story flat roof in black? Plus some landscaping to soften things?

I agree with you that going gray in Seattle might not be the best choice.........

Using all one color is working in your rendering.

As a response to your reply to, and in agreement with Lydia, you were on the right track with the original concept, but the wood element really should wrap around the right side of the front gabled form of the home. In addition, I would love to see you "group" the windows (the "L"-formation) on the right by possibly introducing a third siding element. Perhaps something like a protruding metallic rectangle "hugged" by those windows - you might be surprised by the effect it has. To answer your original question though... Go with ipe wood if your budget allows for it on the vertical wood element, and stick with a warm grey for the Hardy. As far as spec'ing a gray for you, it would be a disservice to even try without experimenting with colors yourself on-site using natural lighting conditions. If you get too stumped... take several swatches of potential paint colors out to the site along with samples of the other exterior elements (window color, wood siding, roof shingle, fascia color, etc.) and post here; you will get plenty of feedback. Have fun and hope it all comes together for you!