I finally found the time to do a new series of experiments with the red allotrope of selenium. I found the following:
Under the liquid, where it is prepared (containing SO2 as reductor), the selenium appears to remain red indefinitely (at least after so many weeks I do not see any change on my original sample, it still is nice red).
When the selenium is washed a few times with distilled water in order to remove the water-soluble chemicals, then it quickly turns darker. I let the sample dry, but after one day, the dry sample was even darker than red phosphorus, it was really dark brown.
When the selenium is washed with distilled water a few times and kept under distilled water, then it also turns dark quickly. After one day, the sample was almost black.
So, concluding, red selenium is not stable under ordinary storage conditions. Only, when stored in a reducing wet environment it appears to be stable. I have no explanation for this, maybe someone else has an explanation?