sunset: The time of the day during the evening just before the night, when the sun is at or near the horizon and the sky is usually colorful with the red and orange colors.
twilight: Twilight is a time of day when the sun is low in the sky, either at dawn or dusk. Because sometimes you don't really know which it is in an illustration.
evening: The end of the day when the sun sets.
dusk: Dusk is the beginning of darkness in the evening, and occurs after twilight, when the sky generally remains blue. Use this when it's not yet night, but the sun has fallen.
Is it possible to have a better clarification of these tags? It seems logical that sunset could refer to the sun physically being seen in the sky, while twilight could just refer to the color of the sky without the sun necessarily being shown. Evening could probably be the same purpose as twilight. Is there enough of a difference between dusk and evening that would warrant separate tags?