Is your tank in the fridge? If so, even when full, it will read "red". Also, 1x 5# tank should last 4-6 kegs of carbonating and dispensing.
Here's a copy-paste of an earlier explanation of mine:
CO2 in a tank is mostly liquid, with a bit of vapor on top. The gauge is reading the vapor pressure, which you can think of like the pressure that the liquid CO2 is making while trying to boil. As you pull CO2 out of the tank (dispensing beer), more liquid CO2 boils into vapor to replace what you take.
The vapor pressure of the CO2 depends on temperature. So, at 70F, it will read like ~850 PSI, (going from memory here, but you could look these up). At 38F, it will read 500 PSI. It will CONTINUE to read that pressure until the tank is almost empty, at which point all of the CO2 liquid will have boiled, and the vapor pressure will start to drop as you use the last bit of vapor.
What this means is, your tank will stay at 500-600 PSI the entire time it's in the fridge, until JUST before it runs out, (before you use up all your CO2), at which point it will drop very quickly to 0 PSI.
The gauge is not like the gas gauge in your car. It basically only tells you if you have CO2 or not, not how much you have left.
If you don't use your CO2 for 20 years, (don't pull any pints), and don't have any leaks, it will stay at 500-600 PSI that entire time, (if in the fridge), and at any time if you remove it from the fridge it will jump back up to 850 PSI. As you use it, and keep pulling pints, and keep pulling CO2, it will stay at 500-600 PSI until it suddenly drops to 0, at which point you need a refill.