A quiet Greek and you turning very pale (3,4)

'very pale' is the definition.
(ash grey is a pale colour)

'a quiet greek and you turning' is the wordplay.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'greek' becomes 'gr' (abbreviation for Greek).
'and' says to put letters next to each other.
'you' becomes 'ye' (historical version of 'you').
'turning' is a reversal indicator.
'ye' backwards is 'ey'.
'a'+'sh'+'gr'+'ey'='ASH-GREY'