The colored circles indicate the flight category at each airport, as reported by the most recent METAR.
The color coding is the same as on other ForeFlight devices:
Green: VFR. Ceiling greater than 3,000’ and visibility greater than 5 miles; includes sky clear.
Blue: MVFR. Ceiling 1,000’ to 3,000’ and/or visibility 3 to 5 miles inclusive.
Red: IFR. Ceiling 500’ to less than 1,000’ and/or visibility 1 to less than 3 miles.
Magenta: LIFR. Ceiling less than 500’ and/or visibility less than 1 mile.
A question mark indicates that an airport is not issuing METARs.
Image 1. Flight categories shown on the Apple Watch.