Salaam Allaikum,
There may be some slight differences between the marajah, so it is important to check this with your specific Marja, but in general, the time of maghrib begins when the redness in the Eastern sky has dissipated.
Oftentimes, people think maghrib is when the redness of the western sky (where the sun sets) has dissipated but this is actually not the case and Maghrib occurs a bit earlier than this....