From my experience I can assure you that the color will not change if you freeze in test tubes sealed with aluminum foil or cap. Make sure serum is protected from environment and light; then you are safe. You can store in appendrobs too, but here you can face a high chance for occurrence of plastic flow if you store for a long time. It's better to store in glass not in plastic. Enzymes having high molecular weight protein molecules, do not change the concentration easily in collected serum like blood sugar (changes rapidly in serum). You can run the method. But if you wait more (i.e. 2 months or more), result can deviate from the median value.

if you have frozen the liver enzymes in clean glass containers and kept for only 21 days and found loss in activity, remember it has nothing to do with glass as materials, it might have other reasons.