That light indicates a low voltage situation. If it is happening when cold, it is most likely that the alternator is having difficulties. This would indicate a replacement of the brushes (done by replacing the voltage regulator) or a rebuild is necessary. It is easily explained by considering that worn brushes, at the limit of their spring loaded travel, do not make solid contact with the slip rings until warmed up and they expand just a little.
At least that explanation makes sense to me. I don't know if that is exactly the case in your car.
A rebuild is more likely. This is because, to run the inner armature voltage, two slip rings are on the shaft. The brushes run on the slip rings. The slip rings, made of copper, will wear also because the Bosch brushes are very hard. Worn slip rings will not last, even if new brushes are provided with the voltage regulator.
Anyway, if things are checking out when tested, you might want to save up for a rebuild or alternator replacement in the near future. Don't let the battery be continuously discharged if the alternator fails or you will get to purchase a new battery also.