Petals change colour (6)
I believe the answer is:
pastel
'colour' is the definition.
(I know that pastel is a type of colour)
'petals change' is the wordplay.
'change' indicates an anagram.
'petals' with letters rearranged gives 'PASTEL'.
(Other definitions for pastel that I've seen before include "Pale or light colors" , "Crayon - gentle shade of colour" , "Staple (anag)" , "art work" , "Soft shade; crayon" .)