I've seen more and more of these painted rooms lately, where the trim is painted the same as the wall color. You can always make a case for highlighting custom molding in a home by keeping it classic white or differentiating it from the rest of the room. But I really love what this can do for a room too! Especially when it's a darker velvety color that envelopes the room.
DESIGN TIP: It's true that you can paint dark colors in a dark room! When you paint a "light" color in a room that doesn't receive much natural light it actually falls flat instead of brightening it.
When you paint the wall and trim the same color it creates a seamless look with subtle texture from the outline of the molding. It adds just enough visual interest while reducing the amount of visual lines and therefore a "broken" or "choppy" look. This type of paint treatment can help in rooms that lead into others, or don't have much wall space.
And as much as I love the dark colors, it works equally as well in other colors too. When the walls and trim are the same color it really creates a strong background for artwork, accessories and furniture to pop!