Riding a bike at night is not safe to start with, but you can't be scared of everything. The main visual of your bike at night is your lights or any reflectors, not the color. A yellow bike might be slightly easier to see, but blue is a dark color which would react the same as black. Regardless, if someone doesn't see you, it won't matter what bike color you have. Bikers have to drive defensively because most car drivers don't even see them.