Green Alert: This represents 'No Warning', and means 'No Action' is required.

Yellow Alert: This means 'Watch', and calls for administrators to 'Be Updated'.

Orange Alert: This represents 'Alert', and calls for administrators to 'Be Prepared'. 

Red: It's a 'Warning', and is a call to 'Take Action'.  

The IMD says that to decide the colour to be assigned to a given weather situation under a 5-day forecast scheme, a specific matrix is followed, “giving thrust on the probability of occurrence of the event as well as its impact assessment".