If you like dicing then have it. In the highlands there are certainly some who will not wear a bonnet with dicing, but in truth there are no hard and fast facts that I have ever found to confirm,one way or the other,the validity of the stories.Some do very much adhere to the anti-dicing thing, though. As to colour; any one you like, but here in Scotland black, duck egg green(hard to find these days it seems) and light grey seem to be the most popular. Just so you know, bonnets of any sort are rarely worn with the kilt these days here and before certain people start chipping in with their mischievous comments, the flat tweed/tartan cap is very rarely worn with the kilt here, these days.