What color change indicates a leak in ammonia refrigeration systems when using wetted litmus paper?

In ammonia refrigeration systems, the presence of ammonia can be detected using wetted litmus paper, which responds to the pH level of the substance it's exposed to. Ammonia is a basic substance, so when wetted litmus paper is used, it will turn light blue in the presence of ammonia. This color change indicates a leak, as it shows that ammonia gas has come into contact with the litmus paper, thus confirming the presence of the refrigerant.

Other colors like red indicate acidity, which is not relevant to ammonia, while yellow and green do not indicate the basic condition associated with ammonia leaks. Therefore, the light blue color change is the correct indicator of ammonia presence in refrigeration systems.