Florin,
That's a hard one because there are so many kinds of epoxy curing agents on the market. Epoxy resin is all pretty much the same and does not react with air readily. The only additive is usually a reactive dilutent to make it easire to wet out fabric. I have used some high temp hardner that had a distinctive reddish tint to start with. Does it have any chemical names on the container? perhaps TETA, Polyamide, or something like that?
Kevin