dunkelblau
dark blue
Describes a dark shade of the color blue.
Er trug einen dunkelblauen Anzug zur Hochzeit.
He wore a dark blue suit to the wedding.

dark blue
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dark blue
Describes a dark shade of the color blue.
Er trug einen dunkelblauen Anzug zur Hochzeit.
He wore a dark blue suit to the wedding.

'Dunkelblau' (dark blue) is made up of 'dunkel' (dark) and 'blau' (blue). This pattern is used for many colors to describe the shade more precisely, for example 'hellgrün' (light green) or 'tiefrot' (deep red).
Der Himmel war heute nicht hellblau, sondern dunkelblau.
The sky today was not light blue, but dark blue.
When the adjective stands before a noun, the declension ending is attached to the entire word 'dunkelblau' (dark blue). It is treated like a simple adjective.
Sie trägt ein dunkelblaues Kleid und passende Schuhe.
She is wearing a dark blue dress and matching shoes.
'Dunkelblau' (dark blue) is often used to describe clothing, the sky, or the color of water. Typical nouns are 'Anzug' (suit), 'Kleid' (dress), 'Himmel' (sky), and 'Meer' (sea).
Das tiefe Meer schimmerte dunkelblau in der Sonne.
The deep sea shimmered dark blue in the sun.
blue
'blau' and 'dunkelblau' both describe the color blue. 'Blau' is general and can refer to many different shades of blue. 'Dunkelblau' is more precise and means a dark shade of blue, for example like a navy jacket or a night sky. Learners may confuse the words because dark blue is also blue. If the exact brightness does not matter, simply say 'blau'. If you want to make clear that the shade is dark, use 'dunkelblau'.

Für die Bewerbung wählt Mehmet einen dunkelblauen Anzug.
Mehmet chooses a dark blue suit for the job application.
Im Park trägt das Kind eine blaue Mütze.
In the park, the child is wearing a blue hat.