Colour of writing material on piano (4)
I believe the answer is:
pink
'colour' is the definition.
(I know that pink is a type of colour)
'writing material on piano' is the wordplay.
'writing material' becomes 'ink' (substance for writing with).
'on' says to put letters next to each other.
'piano' becomes 'p' (musical abbreviation).
'ink' put after 'p' is 'PINK'.
'of' acts as a link.
(Other definitions for pink that I've seen before include "Rather embarrassed" , "like the Financial Times?" , "Salmon-coloured" , "See 21" , "(Leftish) colour" .)