I'm very tired but I'll try to cover all the areas you need to improve in.
I'm imagining you are quite young because the subject matter and your handling of it is very "juvenile" for lack of a better word.
No one will take you seriously with what you've drawn here. Vary your subject matter, no over buff men and women, try some
different characters perhaps of differing age and ethnicity. Everything is very contrived and one dimensional
Now onto technicalities. Your line is bad, its purposeless, scratchy and without decisiveness. A lines purpose is not to outline the boundaries between areas of form, it is to
articulate the recessions and protrusions of form along with the weight and stiffness of form. A fleshy bicep swinging through the air clinging by a tendon and folding in the brachioradialis requires a totally different treatment than hair or cloth. Study how top animators handle this or
https://encrypted-tbn3.gstatic.com/i...fA0KCpsEwL0x_g
illustrated here by the best portrait artist to have ever lived (imo). Notice the fleshiness of the lips and cheeks.
Forget about colour until you can draw well, it takes 20 years to learn to draw it takes 20 minutes to learn to paint, obviously exaggerating here, but until you understand form and shape (drawing) fucking forget colour.
Your anatomy needs a LOT of work. Forget about drawing from imagination until you can name and place the origin and end of every muscle in the body from memory.
Proportions seem inconsistent because I can't see a full figure. It generally looks like you are operating within the 7.5-8 head rule, and I can't imagine you plan that so you
probably have a good eye actually, the arm length is way too long in some of these, but the crest of the rib cage isn't considered at all so that doesn't matter.
The "shading" looks more like colouring imo, its not "this is lit from here and this is the refraction of the light and its intensity" its more "dark here, shade" a lot of beginners make this mistake in drawing, personally I would just focus on line for a while, keep things simple.
This may seem like a lot, but what I'm illustrating to you is that if you are pursuing art as a hobby thats great enjoy it and improve at your own leisure. If you want to pursue it more seriously, strongly consider pming me so I can steer you in the right direction because as you are heading you won't get far.
Sorry if thats all over the place, but I'm very tired.