I find grey to be a vibrant color, mostly for me since I'm a huge fan of older black and white movies. If the color grey exists as an inbetween for the two. Introducing shading, lighting, tone. Look at pencil and pastel art as an example. If I fool with pastels its usually on tan parchment with black, white and grey colors; but that's just me and my opinion on it. Those are interesting pictures you've featured too might I add.