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I think the second one was a bad example. Maybe these will give you a better Idea of what I mean.
In this image, I think you've got two things working against you. I see what you mean by "gray," and you're also competing with the very bright (white) background object between the heads of the closest and second-closest girls. That object intrudes considerably. The color version benefits from the distinctive aqua color of the dresses, which provide a solid cue to the eye that a face will be sitting on top of that column of aqua. So in this shot, in particular, I think the color version is a better option.
In the original images in the thread, the first pair are just low-contrast images, so they're going to look gray. But I actually think they work better in your B&W conversion than in color. Like those a lot.
As for the next image of the small group in the alcove -- I see what you mean, sort of: the faces are just a bit hard to separate from the other material, and that probably has something to do with local- or overall-contrast levels. But it's not bad. Add a bit of clarity/structure to that one and you might be happier with it.