Muted (3-3)
(Other definitions for low-key that I've seen before include "Lacking intensity" , "Subdued" , "Restrained, not intense or emotional" , "Unemphasised" , "Modest, restrained" .)
'muted' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pale that I've seen before include "Light-coloured" , "Upright member" , "beyond which one is out of line?" , "Not bright - limit" , "Containing little colour" .)