Here are the clues:
Set in France.
Was Radio 4 book at bedtime about four years ago.
The story concerns a painter who has to travel a great deal to collect materials for a particular dye/colour that he uses for his paintings.
The title of the novel is the name of this colour/dye.
The painter has an affair with someone while he is on his travels.
Those are the clues.
If anyone can work out what the book is then they will win a special prize...