Correct me if I'm missing something... but when has 'less popular' ever immediately meant ugly or due for disregard? Tail colors that aren't common are not as popular as colors that are used often. It's synonymous, and having a tail in a less popular color does not mean that anyone is disregarding your colors. They just aren't common. I'd take it as a compliment, I for one love unique tails.

Anyways, to answer the question I don't see greyscale tails very often. I've seen a couple of shark tails that look fantastic, and Mal's tail is silver.
Earthy tones are common in nature but not in tails. Browns, earthy greens, tans, etc.

I'd love to see more neon/phosphorescent/glow in the dark tails!