Chime in, but some ways to describe the derrière. It is a huge part of my business, and I'm learning it does have a lot to do with turn ons for guys.

Skin complexion or skin type? Color?
Chocolate with a touch of cream
Exotic / dark tan
Super pale
Pale & freckle kissed
Smooth
Soft
Bumpy
Saggy (of course we don't want that lol!)
Etc etc.

Just some quick ideas. I know our skin behaves differently week by week, so we would want to avoid the opportunity to be careless about it, while still giving an accurate description

The review process while not complicated is time consuming. Filling in any more categories will make it more arduous and some of your suggestions are too ambiguous and subjective. These items have to be filled in each time we submit a review. It is a pain in our asses, not yours.  

If you want to highlight your ass, have your photographer zoom in on it. The skin tones are subject to changes and again most providers have ads and websites to describe their skin tones or whatever else suits you for the day.

1) Flat.
2) Boyish.
3) Juicy.
4) Rock hard.
5) Ample.
6) Takes Two Hands.
7) I Like Big Butts and I Cannot Lie.
8) Just plain huge.

Just kidding, but if we get any more items categories added to the review process then we'll have to start scheduling 5 minutes at the end of every appointment to sit down and go over the checklist line-by-line.

Nothing says GFE like asking afterwards about what she didn't do with you.  Seriously, that's usually part of the whole break-up fight in civvie land!

That being said, I love a shapely ass and will look for pictures of it rather than depending on someone else's review.