I'd paint the same colour as door and window but fill it with brightly coloured flowers and foliage to add contrast. I think that will look great.

This is a 115 year old house we bought 10 years ago as a rental and just started using it as our summer place...love it so much,we are selling our home in another location and retiring here next year.....

Window color or white seem like they would both be lovely, as would black or black metal. I think that's a great idea and will add much to your patio. Try a bigger pot or pots under the window, as well. (We're also in our "weekend" house fulltime and just building a surround for our too small individual boxes.)

From what I can tell your front door is grey, am I correct? If so, matching the color to a window box would be lovely! Another color that goes well with your home is a deep blue color. Composite PVC window boxes take paint extremely well, as they do not absorb the moisture that could cause paint to peel or crack.

Painting instructions: http://www.hooksandlattice.com/painting-pvc.html

Hopefully this has been helpful!!