Due to the openness of your rooms I would paint all 1 color. Use the color card that stonington gray is on to get the darker color for the niches. When you ask what colors complement it, do you mean for furniture, rugs, accessories, etc?

I would paint it all same color, because there are a lot of architectural details. Light gray like Stonington gray would be good.

All one color or ask the paint mixer to do lighter shades or darker shades of the same color by adding more or less white to the formula. Check Revere Pewter from BM

Select a neutral color for all the rooms and use your accessories to change colors for each room like you see in the photo below where the walls are white and the blue accessories supply the color for the room.

With so much classical architectural detail, consider Ben Moore's Navajo White for walls and White Dove for trim. You'll need to change the wallpaper in the niche and whatever is in the recess above the fireplace as well.

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Painting all one color would be a much more modern look for your space, Stonington Gray is a good choice but I would keep the trim white and the ceiling a 1/4 tint Stonington Gray in a flat sheen.