The still lifes look pretty good already! Also your colours seem to be pretty nice already. As you asked what to study further, I'd suggest faces. Study the structure, the planes of the face. Also pay attention to your edges. In the pinup studies they look pretty nice, but in your faces it seems like you're concentrating too much on the contours of each feature. Try to blend them more together. As weird as it sounds, but keeping the contours of the eyes and mouth soft makes them look a lot more realistic.
Keep up the hard work!