So these studies were done in a method that most artists would be afraid to do. Usually art teachers will teachers you 1) to draw from life, 2) if you copy a photo, don't copy it exactly - make sure to edit and design the shapes 3) don't color pick in photoshop.
I friggen traced photos in photoshop, and color picked the heck out of those photos!! They were done super quick in like 30 minutes to 2 hrs. There were a lot of benefits from doing this.
1) You learn a lot from tracing. I really like artists like J. Scott Campbell and Bruce Timm - but all their girls and poses look the same cuz of their habits and muscle memory. Tracing teaches you new habits because you aren't really drawing with your own "hand."
2) It's quick and pleasurable! It's really exhilarating to finish up a nice painting in 30 minutes. It makes me want to continue and be persistent in the studies. If I were to do it traditionally, I know it would take me at least 4 hrs for a nice full color study. If I were to do a traditional 30 minute gouache painting - I wouldn't capture any of the detail. But I think the detail is the best part! The detail expresses the fashion and style of the model.
3) Subtleties. I layer the reference on top of my "painting" and I toggle it on and off. It trains my eye to be very discerning and recognize the subtleties and nuances of the photo. When I'm free painting it - I think everyone kind of goes back to their habits and tricks to make it look good. But when you are copying the photo dead on - there is a rigidity there that really trains you and makes you stronger.
I draw in my sketchbook and paint on traditionally as well. I just think this Photoshop tracing technique is a great way to compliment traditional studies.