A period of time at the end of day when the sun is below the horizon but before the full onset of night, especially the darker part of twilight.
"The couple walked hand in hand through the quiet streets as dusk deepened into a soft indigo above the city skyline."
become dusk
"As we walked along the beach, the sky began to turn a deep purple and the temperature dropped until it became dusk."
To begin to lose light or whiteness; to grow dusk.
"The sky began to lose its brightness as dusk settled over the valley, turning the horizon a deep shade of purple."
Tending to darkness or blackness; moderately dark or black; dusky.
"The storm clouds rolled in, turning the sky a deep, dusky shade of purple before night fell."