The first thing that jumps out at me is the costume colors are not completely correct. Captain America's "briefs" are blue, not red. Keep that reference handy!
The first thing you need to do when looking at the lineart is define the light source. This is usually established by black, inked core shadows and line weight (thin lights are facing the light source, thick are opposite it). Marcio set up a light in the top right, so all your rendering needs to follow that:
Stop choosing colors by adding white or black to a base color. This book is a great introduction and instructional resource for color and light in relation to art and illustration:http://www.amazon.com/Color-Light-Guide ... 0740797719
There are also great light and color theory resources online, google away:http://www.itchy-animation.co.uk/light.htm