A green roof pairs well with gray and white homes. For a subtler look, pair a green roof with a siding in a lighter shade of green. I think you are on the right track .You have chosen harmonious shades . New paint doesn't have to match existing colors, but it should harmonize. Some general tips are- Green roof - Gray, green, or white house colors. You could use a dash of yellow also.

Sophisticated start. You could go with black and white photography, architectural pieces, art pieces- anything you love on the wall. Then, add pillows in the colors of the art work. If you want to stay more monochromatic, think in terms of textural interest, such as a shag area rug, pillows with seam detailing, etc.

I was about to say grey until I read the last line. What about a light brown carpet?

You are so right white is never out of fashion. I would set your budget and then look at your options. If you plan to stay in the house for many years then spend to the top of your budget (if that is going to give you a life time of pleasure), if you plan on being at the home for only a short time / few years then I would go mid range so it looks lovely, will have good resale value but you can invest your well earned money to go towards something else as well. Obviously Marble / Granite is the top of the range but there are so many cheaper and very attractive options these days that look just as good.