The colors are based on the U.S. Environmental Protection Agency’s (EPA’s) Air Quality Index (AQI) color scale, and display the AQI level that would result if measurements were made over a 24-hour period.
The AQI provides information on how polluted the air is and what you can do to protect your health if you plan to go outdoors. You can visit https://www.airnow.gov/aqi/aqi-basics/ to learn more about the AQI.
Figure 1. This table shows what the different AQI colors mean. Source: https://www.airnow.gov/aqi/aqi-basics/.