Yellowing is normal. That's just the nicotine oxidizing from contact with light and/or air. It's not a problem.
Red coloring would most likely mean the nicotine is breaking down into nornicotine or the supplier actually used it in the juice instead of or in addition to pure nicotine. In either case, I'd toss the red stuff as it's either gotten contaminated or was poorly made in the first place.
Take the other out of the freezer and see if it turns red too. If it doesn't, it at least eliminates the juice as the problem.
I'd also lose the juice bottle and get a amber glass chemical bottle to store your nic in. Preferably one with a self sealing cap so you can use a syringe to extract what you need out of it without having to open the bottle and exposing it to air and contaminates.