If the adsorber discolors from above, this indicates moisture escaping from the system. The adsorber is designed as an aerator and deaerator. Thus, incoming air is dried and this dry air is used to regenerate the silica gel as it flows down through the adsorber from the top.
However, if the air present in the tank, or even the liquid, have a certain amount of water, the adsorber will absorb this moisture – the color will change from the top. This will always be the case if an aeration dryer is installed right from the beginning, but only after several months (or years).