You've got a solid grasp of inking to give some shading, just need to practice your brush control some. Brush control comes with time and practice (as you paint, you'll have fewer bobbles, I.E. getting paint from one surface where it belongs, such as the rim of a shoulder pad, onto an adjacent surface where it doesn't. Such as the surface of the shoulder pad). Also the right kind of brush can help with bobbles while painting. I'd do some research, get some small brushes for the fine detail work such as eyes and other small details.
Couple of points I can offer that will help you I think.
First, and I know it's drudgery of the highest order, but try to remove the mold lines from your miniatures. Some people just say 'feth it' and don't take this step. If you're one of these people, then please disregard this advice. I for one find my models look tremendously better with the mold lines removed, but that's just my personal preference.
Secondly: Paint parts before you final assemble them. For instance, assemble the torso and legs together to form one piece, then paint the torso/legs, head, and arms/bolter separately, then assemble as a final step. (I realize some of your marines are push-fit from the
AOBR box, but the advice still stands for those too, just fewer parts.

)
I use a little bit of blu-tack to cover the contact points at the arms and neck when I basecoat/primer the miniatures with spraypaint so I still have a good bare plastic-to-plastic contact point for the plastic glue.
Focusing on one section of the mini at a time tends to help me focus and also not have to fight to paint around an arm or weapon.
I hope these suggestions help you out and keep painting! I like the color scheme overall.
Take it easy
-Red__Thirst-