Colour in character in picture books (6)
I believe the answer is:
tintin
'character in picture books' is the definition.
(character in comic books by Herge)
'colour in' is the wordplay.
'colour' becomes 'tint' (tint is a kind of colour).
'tint'+'in'='TINTIN'
(Other definitions for tintin that I've seen before include "Herge's cartoon hero" , "Herge's adventurer" , "Herge's adventurous young Belgian reporter" , "cartoon character" , "Comic character" .)