Es geht wohl um Farben, die zur Dekoration / Verzierung dienen, im Gegensatz zu solchen, deren Hauptzweck der Oberflächenschutz ist.
That's only partly right. Trim includes the decorative pieces that are used to "finish off" a room. Door trim, window frames, windowsills, baseboards, cornices, etc.
Trim paint is the paint that is used to paint that (and thus not the wall).
Here's a definition (from an informative article).
Trim paint is a category of interior paint that is formulated specifically for use on trim. It has a satin, semi-gloss, gloss, or high gloss sheen. Trim paint usually comes pretinted in bright white and in base colors that can be custom-tinted. It dries to a very hard finish that has some resistance to moisture, and it usually has ingredients that resist yellowing or sagging.