Turning quite sour and blue-green (9)
I believe the answer is:
turquoise
'blue-green' is the definition.
(I've seen this before)
'turning quite sour' is the wordplay.
'turning' indicates anagramming the letters.
'quite'+'sour'='quitesour'
'quitesour' is an anagram of 'TURQUOISE'.
'and' acts as a link.
(Other definitions for turquoise that I've seen before include "Shade of blue - gemstone" , "Birthstone for December" , "Greenish blue; gemstone" , "Greenish-blue colour" , "Blue-green" .)