I assume you mean you're going to paint it to look like glass? I use Duncan paints and the colour I use for faking windows is Wedgewood Blue (colour number OS458). This is a slightly grayish light blue colour (if you do an Internet search for "Duncan paints" you will find a colour chart and then you can pick or blend a suitable colour). I then put a suggestion of reflected light in a corner of the window with a lighter colour.
If you want to paint curtains and stuff in the windowsill, do that first and then wash the window colour over it.