The deep blue in this painting is very beautiful.
青 is the general word for 'blue', while 深青 specifically means a dark or deep shade of blue.
Standard kanji spelling for the color 'dark blue'.
Alternate kanji spelling using 蒼, which can also mean 'blue' or 'greenish-blue'; less common than 深青.
Compound of 深 (fuka, 'deep') + 青 (ao, 'blue'). The reading ふかあお is a straightforward combination of the kun'yomi readings of the two kanji.