I'll let you in on my system. The Official Winn Rules For Bugs Red bugs are worth three points. White bugs are worth two points. Black bugs immediately set your score to zero. All other bugs are worth one point each.
Convertibles are worth double, except for black ones.
Rolls-Royce hooded bugs are worth double, too. Calling a bug that isn't there makes you lose the number of points that would normally be granted. For example, calling a blue bug that isn't there makes you lose one point. Calling a red convertible bug with a Rolls-Royce hood that isn't there makes you lose twelve points. This can result in negative points, and if you drop below zero, a black bug becomes a good thing rather than a bad thing.