Sorry, but is it not their house?

If you're really trying to dissuade them, why not try to find pictures of rooms with dark trim similar to what they're asking for or photoshop pictures of their house with the dark trim.

Ultimately though, it's their house and their decision. They have to live with it and not you. Just sayin...

What type of look r they attempting to achieve? Some looks do call for dark woodwork...Could u show them examples of pics to show your point of view?

Your clients may have practical experience with white baseboards that leads them to prefer dark colors. White baseboards are readily streaked dark by brooms, vacuums, shoes and furniture. Damage shows faster on white paint than dark paint. Or they may just prefer dark trim.

If you are the painter you do what your client ask..they are the one paying...and one who will be living there.

It can work.