Pale colour (3,4)
(Other definitions for pastel that I've seen before include "Crayon - gentle shade of colour" , "Artist's crayon" , "Type of crayon" , "Delicate shade; crayon" , "Pale or light colors" .)
(Other definitions for sky blue that I've seen before include "Colour overhead on a fine day" , "Cerulean" , "Heavenly colour" , "Seeing this, one may be looking up" , "Coventry City footballer" .)
'pale colour' is the definition.
This is the entire clue.
(Other definitions for lilac that I've seen before include "Genus Syringa shrub" , "Flowering tree of the olive family" , "Pale pinkish-violet" , "Pale mauve colour for a flower" , "Syringa vulgaris, light purple in colour" .)