Each venturi barrel has a different FiO2.
The colours represent different FiO2 values, which are also printed on the barrel along with the required minimum flow rate.
Always double check you are using the correct barrel.
For patients with a respiratory rate of > 30 set the flow rate above the minimum to compensate for the increased inspiratory flow - this does not change the FiO2.
Blue
FiO2 = 24%
Flow rate: 2 - 3L/min
White
FiO2 = 28%
Flow rate: 4 - 6L/min
Yellow
FiO2 = 35%
Flow rate: 8 - 12L/min
Red
FiO2 = 40%
Flow rate: 10 - 15L/min
Green
FiO2 = 60%
Flow rate: 12 - 15L/min