How do 35mm film scanners handle the film's base color?

I would like to create a stencil for scanning 35 mm film with my digital camera (similar to https://www.thingiverse.com/thing:2789457 or http://www.frogymandias.org/3-d/index.html ). ,

I have seen several similar scanners on the internet and as far as I know, this is the same principle that enlargers use to transfer images from film to photo paper. So I think that this method should work.

But there has to be something I miss. All these devices send light by the movie and that means by the film base. All the movies I've seen have a brown transparent base or a gray opaque one.

How is it that all this can work? I would expect to either have a brownish tinge over the resulting image or not see anything at all. Where is the mistake in my reasoning?