Question: what is the difference between black and green o rings?
Answer: They are colored blue with an external coating to distinguish them from black nitrile O-Rings that are identical in appearance yet not recommended for use with R-134a. The green O-Rings are Highly Saturated Nitrile (also known as Hydrogenated Nitrile) and are commonly referred to as HNBR or, in some cases, HSN.