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R12 is a colorless gas. Comes out as a white smoke. R12 Oil is just a mineral oil - kinda yellow. A green dye may have been added to the system previously to troubleshoot for freon leaks. It does not come with dye from factory, but it is a common practice to add the dye when searching for leaks.
Why does it come out from the back of compressor - it has an over-pressure valve that protects compressor from self-destruction if the pressure builds too high. Most likely reason for too high pressure is a blockage somewhere in the high-pressure line. Crushed high-pressure lines is one distinct possibility, blocked filter-drier or evaporator valve is another.
Check the high pressure lines visually, check front condenser for blockage (does your engine tend to overheat?)
Dont try to run A/C until you found and fixed the problem - you risk destroying the compressor and you are losing freon.
One method of finding obstructions is checking the high-pressure line temperatures as you go from compressor to condenser, filter-dryer and to evap valve. A sharp temperature drop will indicate a point of obstruction. Unfortunately it requires running the A/C - something I would not risk doing on my car...
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