na-adjective
Describes a rich, dark shade of blue, often used for fabrics, paints, or natural scenes.
彼は濃青色のスーツを着ていた。
He was wearing a dark blue suit.
夕暮れの空が濃青色に染まった。
The evening sky turned deep blue.
青色 is a general blue, while 濃青色 specifically means a dark or deep blue.
Compound of 濃 (dark, deep) + 青色 (blue color).