The only criteria gas that is colored is:

Nitrogen dioxide is recognized as the only colored gas among the criteria pollutants. It appears as a reddish-brown gas and is often visible in the atmosphere, especially in urban areas where it is a product of combustion processes, such as in vehicle emissions and industrial activities. This visual characteristic helps in identifying air pollution levels, particularly in smog scenarios.

Although other gases like ozone, sulfur dioxide, and carbon monoxide are serious air pollutants and have significant health impacts, they are not visibly colored. Ozone is a pale blue gas but usually does not present as visibly colored in typical ambient conditions. Sulfur dioxide is a colorless gas and carbon monoxide is also colorless, making nitrogen dioxide distinctive in that it can be seen, contributing to its identification and analysis in environmental studies.