Heya! Welcome to DW.
You posted the stock photo you used on DA, please make sure you share it here, too!
http://catiaffs.deviantart.com/art/S...ck-3-115011270
If you use a reference and you don't tell folk, they might feel cheated. So, it's always best to share your sources.
It's a great expression to use for Harley- she's oozing attitude.
If you want to get into comic book illustration though, you might benefit more from your painting exercises if you eyeball/copy your reference, rather than tracing. At the very least, use the grid technique, if you don't want to just eyeball it. This way, you're working on your eye-to-hand accuracy, while also producing art!
One last point that might help "level up" your colouring is to make sure you
don't use neutral grey in your artwork, and add colour to your highlights and shadows. It helps make your art look less flat.
I hope that helps! Again, welcome to DW.