[David Lyga]

Part B is the nastiest part the the RA4 process. SOMETIMES if black it can be used but only SOMETIMES. If it is not COMPLETELY black there will be a grace period involved. But, to ALL: please make certain to keep this B part COMPLETELY airtight, either in PET plastic or glass, filled to the very rim of the bottle. It should NOT turn dark. Also, Part A must be kept airtight. Part C CANNOT be kept in PET plastic because it will eat at the sides of the bottle. That is due to the extreme alkalinity of Part C. (NOTA BENE: to avoid confusion: Part C of the C41 CAN be kept in PET plastic but NOT the Part C of RA4)

Of course, if all three parts of RA4 are mixed into your working solution that solution can be kept in PET plastic. - David Lyga

You know, Polyglot, you just might be right: "Strangely, this stuff is reported to keep better at replenisher-strength, ie mixed up, than concentrates."