I have today added an article that explains how you can simply "tint" the visors in Grand Prix 4, as they are in real life. This means the visor view in general will be slightly darker than the T-cam view. Also, if you drive as Alonso, Yoong or Bernoldi the view will be tinted to match their visor colour (i.e. green and orange).
It's certainly a good tip, and adds to the realism of the game. If you are interested, please read the tip (and instructions on how to do it), right here.