No, it is not legal to have a fire extinguisher in your aircraft if its pressure gauge is not in the
green (operable range).
Why?
1.​ FAA General Maintenance Requirements​
○​ Under 14 CFR 91.205 (Instrument and Equipment Requirements), a fire
extinguisher is required for certain operations (e.g., Part 135, some Part 91
aircraft).
○​ While Part 91 does not specify exact maintenance requirements for
extinguishers, you are still required to maintain all required equipment in
operable condition (14 CFR 91.7 - Civil Aircraft Airworthiness).
2.​ FAA Advisory Circular (AC) 20-42D​
○​ The FAA recommends following NFPA 10 for fire extinguisher maintenance.
NFPA 10 states that an extinguisher with a low-pressure reading must be
removed from service.
3.​ NFPA 10 (Standard for Portable Fire Extinguishers)​
○​ NFPA 10 states that if the pressure gauge is outside the green zone, the
extinguisher is considered unserviceable and must be replaced or
recharged.
○​ If the extinguisher is rechargeable, it must be serviced by a qualified technician.
Bottom Line:
If the pressure gauge is not in the green:​
✔ It is not considered airworthy.​
✔ It must be serviced, recharged, or replaced before flying legally.