JO 7110.65 3-2-2

Direct a general warning signal, alternating red and green, to aircraft or vehicle operators, as appropriate, when:

NOTE-

The warning signal is not a prohibitive signal and can be followed by any other light signal, as circumstances permit.

Aircraft are converging and a collision hazard exists.

Mechanical trouble exists of which the pilot might not be aware.

Other hazardous conditions are present which call for intensified pilot or operator alertness. These conditions may include obstructions, soft field, ice on the runway, etc.