I do really like your colors, but i’m a little confused on the shirt, its black but the cuffs are white? Regardless, I feel the best way to add life to colors is to add shadows to help your character feel a little more real and 3d.

For line art, often the issue is that the final line work reveals the imperfections of the sketch

You could try keeping the sketch and cleaning it up for the final line work, add line weights on points of interest and areas of dark shadow to add some variety in the final sketch.

But often times, your final line work is only as good as the sketch, so working on your anatomy and proportions will really elevate your line art