It's a pity your windows were installed when they were because modern frames can be supplied with brown on the outside, and white on the inside, which is far better because white will relfect in more light. A couple of years ago a neighbour painted their UPC garage door white. I don't know what paint was used. It was just lightly rubbed down and washed with soapy water and then some kind of white paint used. It is exposed to all the elements and so far looks perfectly OK so it must be possible to do it. Maybe if you contacted one of the Customer Service department of one of the main paint manufacturers like ICI Paints, they may be able to give you some helpful advice on the subject.