A. This is a question I would have answered differently a few years ago. At that time most queries or correspondence with editors happened by mail. These days you will find that most accept and expect e-mail queries and responses to their questions. For that reason, if they print out the queries that interest them they will all look like “plain vanilla.” Some try to overcome this by adding photos or cartoons or some kinds of illustrations to their e-mail queries. Don't (unless those illustrations will actually be part of the proposal). Such ploys are seen as unprofessional by the editors and actually turn them off—rather than drawing their attention in a positive way. There is only one thing that makes one manuscript stand out over another—quality of the writing and strict adherence to their guidelines. If you manage to succeed in offering such qualities, the next consideration is appropriateness for their audience. If you score high in all three of those areas, you will likely make your sale.
As far as selecting your stationery, most writers today simply buy a better quality paper and create their own letterhead on their computer. If you don't want white for the stationery, choose a muted color, such as gray, tan, or cream. Again, using bright colors or pre-printed stationery in cute or wild designs also come across as unprofessional. Just send them a dynamite query.