Pretty good start I reckon! Maybe try some value studies of compositions?
Okay, I did some value studies of Caravaggio pieces! Cause he's the master at contrast. I real quickly threw some colors on them just for fun.
With Doubting Thomas, I only used 4 tones. Black and white first, then light and dark grey. I tried to keep the reference no larger than a couple inches on the screen so I wouldn't get caught up in detail.
Doubting Thomas was about two hours, John the Baptist about 1.
You've got some interesting stuff going with your various digital painting styles - if you're new to it, it's good to explore different styles like you are, figure out what works for you.
I really like the face speed paints in the second post, I think they're some of the strongest work here. Nice colours, good separation of planes/light/dark - could probably stand to have more contrast, but they've got nice hues. When you start to use lower opacity brushes (like a lot of the ones in the next post, especially the girl leaning on her arms) you start to get streaky, and begin to lose the form, as well as gaining texture that you don't really want. You can really go two ways - fairly sharp separation of planes, or soft, smooth transitions (or combine both, depending on the type of shadows). Try blending a little - smoothing out those transitions.