What conditions can cause an oil to turn green? Oxidation?

Ingersol/Rand pump with GST-32 oil. Slinger ring is generating a lot of brass particles. The acid number in low and H2O is <10ppm. New GST-32 is a pale yellow, the oil in the pump turned a dark green. Very low ferrous material. Will brass oxidize in oil?

I would be alittle concern. But im not a pro. I would recommend testing your oil for coolant. if i remember there are two ways to do it. one is to put your oil a clear plastic bottle. see if the water or the oil sinks. i forget which sinks. but if you see two layers of different color.( u gotta let it settle). another way is to get a hot frying pan or iron and drop the oil on it. and hear if it sizzles, becasue water sizzles.
if you get a conclusive test of coolant being in your oil. u need to get it worked on./
I'm not a pro, but that's just what i would do. im sure there are other ways

Pete, have you analyzed for dissolved copper? The brass has copper in it and that may be the source of the green color. I see bright green oil on occasion in refrigeration oils when the copper has been put into solution. I would expect copper levels of at least 30 ppm to give the color you described. You say it has low acidity, but what is the actual TAN value? TAN of 0.1 may be high enough to support dissolving copper.

It may be a good thing you don't see iron. Copper plating may be possible when the copper in solution deposits out on a freshly exposed ferrous wear surface, and is replaced by iron in solution.

I don't know much about refrigeration oils but I have seen the dye in grease that is in bearings purge into an oil sump turn the oil in the sump green on some agitators. A lab analysis should be able to indicate if it is metal and might identify other possible chemical reasons for the change.

Any chance of it being a "trace element" added to the oil? (Green/fluorecene)

Bacteria can also cause a green color, but that would usually require the presence of water.

Just an update:
Cu= 81ppm, we put a sample in the centrifuge and it looks like the Comstock Lode w/ all the yellow metal in the bottom.
DR Ferrography WPC= 7.4
Analytical Ferrography: the slide looks like a brass highway w/ severe cutting wear.
Thermography shows the pump/bearings are running well below our max. temperature.
We suspect the slinger ring is taking a pounding in the pump.
This pump is having vibration issues, degraded pedestal.

I just haven't seen oil turn this dark green before. I suspect the oil turned green due to all the copper particles in the oil coming in contact w/ air. Am I off base w/ this theory?

Pete, you seem to have the right answer. Copper oxide (cupric oxide, CuO) is used as a pigment to color glass green. Other types of copper oxides are red (cuprous oxide, Cu2O).

At the San Onofre Nuclear Generating Station we have seen various shades of "green" oil. The light green oil is typically caused by large amount of ferrous debris (rubbing wear), typically measuring a high Wear Particle Concentration. We have set the sample on top of a strong magnet and removed the ferrous particulate, which restored the oil to the new oil color. The dark green oil typically comes from brass; ie copper, zinc, lead. The brass is normally from a slinger ring break-in due to improper ovality. When you inspect the slinger ring, be careful of sharp edges, this is a tell-tale sign. Recently we recieved a new GE motor from Brazil and found a large amount of brass in just a few hours of initial run time. We went into the bearing housing and cleaned it, and it hasn't come back. A standard bearing flush will not work. You will need to remove bearing covers, drains, remove oil sample valves, etc, and use a solvent to clean all surfaces.