I would match the trim with the house blue/gray color.

&darzy I wanted to do a blue/gray but there's nothing blue/gray in my house

If you wanted to give it a little more pop you could pull a "brick red" color from the bricks on your house... similar to below (pictures are a little dark so it is a little harder to see the tones).

Or

I'd want it to match also. I personally wouldn't want a shed to pop with color and be a focal point.