The red zone is an indication that the pressure is getting low and the cylinder needs replacement for its intended purpose eg putting fire out.
As to when it needs replacement when used in you CO2 injection depends on what regulator you have.
If it's a single stage reg don't wait till it gets to low as when the cylinders pressure gets low it may do what we call an end of cylinder dump eg it dumps the last CO2 very quick and can lead to s very low pH in tank and high [CO2] which can be fatal to the livestock.
Two stage reg you can use it till there's no CO2 left as two stage regs don't suffer from the end of cylinder dump.
So all depends on what reg you have.