because, that was the name of the product. (movie,book,etc) if
the main subtitle was "the red violin" the author will base it on
that name so the violin has to be red.
The answer to this question will ruin the surprise ending of the
movie - which is that the maker of the violin in the movie used his
dead wife's blood to varnish the instrument, and in doing so the
violin took on her life force.
Which is complete rubbish since dried blood turns dark brown
when dried.