Color to black and white?

I don't understand why the film is sometimes in color and other times in black and white.
It's not like it's at different times in the character's life. Sometimes the color switches right in the middle of scenes.
For example early on in the film the cousin arrives and she takes the family to a restaurant. The scene starts in b&w while they are eating dinner and talking. The piano player starts playing and a woman is singing along, the family turns their attention to the piano and singer. Then the film becomes color. Then b&w, then back to color. All within about 10 seconds. Then it stays in color for a wide shot pulling back showing the family at the table and the woman at the piano singing. The scene ends - fade to black (LITERALLY! Because the next scene is in b&w again)
I'm all for different styles of filmmaking and I can even think of a few films that switch to black and white from color. But usually it's to signify a different time period. Or a flashback. Or sometimes it's a stylistic choice but it somehow makes sense (Tarrantino has been known to do this). This made no sense to me and I only found it to be distracting and annoying in a film that is otherwise quite good.