It's not a sin to paint this particular detail the wall color. There's plenty of other trim. Personally, I've never been a fan of the chopped up look, and the proportions that result : )

I would paint it the wall color. You can use a slightly different finish - typically trim is painted a semi gloss or just slightly more of a sheen than the wall itself. Then paint the baseboard, crown, and door and window casings whatever color you choose for your trim throughout the house. Another option would be to paint the ceiling, crown, and upper portion of the wall, including the picture rail a color and then paint

the lower portion something else. See the attached photo of a little girl's room I just did. Lastly, you could paint the whole wall including picture and chair rail, and then paint the crown, window, door trim, and baseboard the same color- something darker. That's a more dramatic look. It depends on what you are going for in here! If it were me- I'd do all white! I'm leaning away from greys and towards whites these days. congrats and good luck!