COVID-19 National map.
See also national time lapse and State maps.
1) Colors are like a traffic light. Green, yellow, red... but then violet, purple, blue for even higher infection rates.
2) Colors represent number of active cases proportional to a county's population (per capita). This is only active cases (new cases in the past two weeks) and is not total cases in a county. This is different than other maps which tend to show total cases since the beginning of the epidemic.