When designing structures, it is best to design with the whole in mind. Therefore it is best, when asking about exterior finishes, to show all exterior elevation and not just the rear elevation.

How far are you into the design process?

On your drawings, the gabled peaks seem to be shingle siding, is this staying? Do you envision that area being the same color as the rest of your house? Curious as to the three front entry doors?

Choosing between board and batten versus the lap siding is a personal decision. Uncertain of the style you're leaning towards, I would want to keep the facade the same color, especially with the wood elements.

Also, not sure how much you're able to change and this might be a moot point, but the three front exterior doors are throwing things off to me. Since the middle door is given the most prominence with the gable, this logically reads as the main entrance and where guests would enter. The one on the left, seems out of place. I would want two windows mirroring what is to the right of the gabled door space. The perception of the very far right door is of a garage/utility door, except that it has sidelights and a transom. The little window next to the garage looks a bit squished. Just my 2 cents :)

The darker the metal roof, the more fading it will experience. My green metal roof is 10 years old. We had to make repairs around the chimneys and it involved replacing with new panels on each end of the roof. You don't realize how much it fades over time until you get to compare it to new material.

I would suggest silver or light tan for the roof. It won't fade as much and will reflect the sun.