I have 3 small points to add.
First, responding to the original question about visors.... chose a colour (or colours) that actually do something for your visibility, and worry about fashion later. grey is best for bright sunny days, yellow is best for dim, cloudy days, and blue or green are a good comprimise for mixed light conditions.
As for sponsorship, first of all, if you don't have a log book, any effort you put into cover letters is a waste of time. Sponsors need to know what specific events you have participated in, and how you placed. They will consider sponsoring you as an advertising expense, so just like any other ad expense (radio or TV, magazines, internet, etc), they need to know that there will actually be an audience to see their logo, and that said audience is within their target demographic. Your log book will be the proof that you are worth spending money on, and without one they wont even give you a second thought.
Second, be prepared for disappointment even when you do start to make progress in getting sponsored. The first sponsorship deals you are likely to get will be nothing more than a discount on the products that the sponsor sells. For example, you might get 10-50% off on tires if you agree to put the tire manufacturor's sticker on your car (and prove that you did by sending in photos of yourself racing). If you are looking for a fully funded race seat and a take home salary on top, you need to be winning championships, not just races.