Just rereading this thread. So much bad information that it is scary.
AC dye will not harm your system. All AC shops use it. Sealant is another story, NEVER put that stuff in your car or you will turn an inexpensive repair into a very costly one.
Do not invert the cans. Doing so charges as a liquid rather than gas. You risk "slugging: your compressor by charging with liquid refrigerant unless you have a machine designed for that purpose.
You should not just "top off" the system. Overcharging can result in costly problems. You need to evacuate, vacuum, and recharge. You can not measure how much refrigerant remains in the system by pressure readings.
Note that you can not legally vent refrigerant into the atmosphere, it needs to be recovered.
While Walmart will sell you a fancy can with a low pressure gauge attached, low pressure alone will not tell you much. You need to measure low and high and then use a refrigerant chart based on ambient temps.
