This is the 4346th most frequent French word.
foncé
Dark.
Here, 'foncées' is used to describe colors, meaning 'dark' or 'deep'.
J'aime les couleurs foncées comme le bleu marine.
I like dark colors like navy blue.
In this sentence, 'foncé' is used to describe the sky becoming physically darker as part of day transitioning to night.
Le ciel commence à devenir plus foncé à mesure que le soleil se couche.
The sky starts to become darker as the sun sets.
'Foncée' here indicates a change in tone or shade due to mixing colors.