Gelules were made with the following composition: Aminophylline 220 mg Promethazine 12.5 mg Prednisolone 6 mg

In my opinion, the brown discoloration comes from the fact that Promethazine decomposes under the influence of the of Aminophylline. I also suspect that Aminophylline is the most hygroscopic product and the part that dissolves in the adsorbed water gives an Alcalisch environment that is very detrimental to the promethazine, a part of which also dissolves as hydrochloride salt. In this light, Promethazine base will be formed, but Promethazine base is not completely insoluble in water! In solution, Promethazine is highly oxidation-prone and discoloration occurs. Normally we add to a promethazineHCl solution Vit C as anti Oxidans.
Several steps can be taken to avoid discoloration:

1. powders Dry well and keep dry; but drying is a problem because of their instability
2. to add the original composition per capsule 100 mg Vit C
3. and previously use theophylline instead of aminophylline.