The colors and tone in this are really strong and the colors work pretty well together. That being said, the red background stands out more than the figures and is pretty hard on the eyes, and makes the hero on the right, who is more interesting IMO, blend in. If these are OCs, what I would recommend is doing just their rough silhouettes in black on a 50% grey background. This will allow you to experiment with their overall visual "feeling".
Another thing is that the muscles are very well drawn, but you could improve them even more by looking at human anatomy books and pictures. Look at the muscle structures, and then layer the skin on top, just like cloth layered on the human body.
Finally, I'd recommend drawing from life from time to time, or at least from a clear photo. This will help with the poses which, while action-packed, are unfortunately this drawing's obvious weak point. Don't draw from another drawing/painting - that is 1.) plagiarism and 2.) not as good for you due to the defined edges making it too easy to tell where to put the lines. Proportions are important to know. Human tends to be about 7-8 heads tall, with the average human being 7.5. Besides the obvious brain-containing head, the torso (below head to the point at which the femur connects to the pelvis, AKA the great trochanter) is 2-2.5 heads. From the great trochanter down to the heel/sole of the feet, it's about 4 heads. Remember, these are guidelines, not the be-all-end-all of proportions.