Refrigerant is a colorless gas or liquid depending on temp or pressure. The sight glass will appear clear when the system is empty or when it full. When it is part full the liquid is foamed by the turbulence of flow with gaseous refrigerant. This will look milky.

To really get an idea of what you are looking at, have someone inside the car shut off the AC compressor while you watch. Even a clear/full system will slow down and show some foam and then the liquid surface level will appear and recede. The only true way to know the state of charge is by weighing in the proper amount. Arthur is sure right about that fan not coming on if the pressures are low.

I get a few cars every year that start to run hot because the AC has lost enough refrigerant that the fan stops coming on and the cooling system gets noticably hotter as a result.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician