If you are coloring for fun and have no interest in becoming a colorist, that's great, and this post is not for you. Color whatever you want! :)
Coloring pinups is fun and easy, and I remember practicing on them when I first got a tablet, but they can't show how well you can tell a story, and that's what editors want to see. That's what coloring in comics is. A pinup is a single image, by the way... like a cover or splash page or just a single drawing. Comic book pages are generally made up of sequential art... pages with multiple panels.
I actually like giving critiques. Weirdo, right? I wish I received more of them early on myself. If you've asked me for a critique, and I didn't give one or you got a very short one, I was either really busy at the time, or you sent me a pinup. NOTE: I can't do critiques for everyone that asks anyway, but sending a pinup guarantees you won't.
There's actually very little one can tell about a colorist from a pinup. As long as your lighting is passable, that's pretty much it. Rendering styles just aren't that important in the big picture.
Can you create focal points on a page? Can you minimize what isn't important? Can you highlight what is? Can you create drama? Can you evoke a mood? Can you clarify scene or location changes? Can you highlight an important moment? Can you create depth and atmosphere? Can you separate your planes (foreground, mid-ground, background)? Are you consistent from page to page? That's what someone looking to hire you needs to see!
Anyway, this isn't a knock on anyone here doing pinups for fun, but they are useless in a coloring portfolio, so why not work on something that CAN go in a portfolio?
And don't mean for this to come off as a rant, but after the 3rd tweet, email, or comment this week wanting critiques on a pinup, I had to reiterate this somewhere! :)