Literary kanji spelling; 瞑 suggests darkness or closing the eyes, reinforcing the twilight meaning.
Alternate literary spelling; 冥 also carries connotations of darkness and the underworld.
Compound of 瞑 (mei, 'dark, close the eyes') or 冥 (mei, 'dark, underworld') and 色 (shoku, 'color'), literally 'dark color' or 'twilight hue'. The exact historical derivation is uncertain; both kanji spellings are literary and rare.