I really don't see an issue with the rooflines. Looks like the angles are fairly similar and the clean lines of both structures should be easy to coordinate, especially in your open, rural setting. Since you've found a stunning deal, I would say, go for it! I definitely would want to find an architect ASAP and by all means, before you pour the slab, make sure you have a plan to coordinate.

Well that is one rule that begs to be broken! I think a matte silver roof would look good....not weird at all. I don't think the house below looks "weird."

I understand going with the grey since it is an 80's style house. You could also do bright bold colors. I do not know if that is your style. I have attached a quick rendering of the gray scheme you mentioned. I prefer light colors above dark colors. This keeps in theme with the sky and earth. Makes the house seem more grounded. The dark entry also draws your eye to it especially with the light coming from the other side of the house. I have also attached a quick rendering of a blue scheme. It looks like you are on a beach and I thought this would blend in with the surroundings. If that is what you are seeking. Hope this helps.

Nearly finished painting my cottage and I am wondering what colour roof I should go for? I could refresh a brighter red (Resene Pioneer Red or Scoria are available) or go for a charcoal gray to match the trim?f thoughts?