I read a description given by Bernard Siu in his build discussion, http://www.maddyhome.com/canardpages/pages/bsiu/index.html, as follows:
Epoxy Color Change (Nitrogen Contamination) Problem --
I decided to use MGS L285 epoxy system with H285F (Fast) and H287S (Slow) hardeners. I bought them from AS&S and transferred them into individual plastic containers inside the hot box (refer to hot box figure). The epoxy is light yellow in color while the hardeners are cool blue.
About 2 days later, the H285F hardener turned reddish brown while the H287S hardener turned to the same color at the delivery nozzle. Since I could not find any discussion on this particular hardener, I posted the question to the Cozy builder’s Group as well as the Technical Support guru of MGS from Canada.
It appears that the hardeners are sensitive to CO2, which will turn brown after short exposure to air. The plastic jugs I was using (above) were permeable enough for the CO2 to ‘sneak’ inside the bottles and contaminate the hardeners. Though MGS tech support from Canada feels if the epoxy and hardeners behave normally, they should be OK to use. Since I am new to this process, I certainly was not comfortable with it. Besides, they looked ugly. I decided to replace them with new ones.
FWIW