The rules can change and be added upon if you wish to with your friends.
Depending on how you want to play the game and the rules you and friends agree on, the force of the punch can vary. Also, you agree with your friends about the varying shades of yellow you will accept such as accepting gold cars. In most cases, the car has to be mostly yellow otherwise it would be considered unfair, allowing even a small proportion of yellow on a car to be considered a "Spotto".
Usually you will have to point out where the yellow car was spotted. In most cases, you will have to believe on the trust of your friends if for instance they say it just turned the corner. Obviously, once someone has claimed to have seen a yellow car you cannot reuse that car as a chance to say "Spotto". But, you can reuse it if you happen to go on the same journey the next day for example. If you punch someone and say Spotto but there is no yellow car, then the person who got hit is allowed to return the favour.
A rule I like to add, which is good considering if you don't like punching people is to call "safe" which prevents someone from punching you as well as you not punching someone.
This game is mostly played on car rides but can be alternatively played while walking or just hanging out with your friends. However, you should note that this game should be played lightly in a country like the US where they have yellow taxis, which end up with you and your friends running around punching and avoiding each other.
Our family's variation excludes the punching; and excludes any vehicle with commercial markings including taxis. Gold, mustard, and lime variations are also strictly excluded -- the point of the game is for recognition to be rare, so that it is a test of long-term alertness as much as anything. Even within these constraints, the number of yellow private vehicles on our roads is increasing quite a bit over the last year or two.