Hapiel just posted THIS about lineart basics for one of our friends in the forums.  It's simple, but gets the point across as to how to clean up your lineart. 

That's much better! Now that you have a defined light source you are finally showing off the form of your subject. There are still a few spots that seem unfinished(the sword and the skulls), but you're on the right track now! Don't be afraid to experiment with colors either. There is a really nice tutorial in the forums talking about hue shifting. I think you would really like the concept of Hue shifting. It'll help make your colors more dynamic. 

It also will help, when coloring, to keep your lineart as simple as possible.  It is good practice to not let it get over 1 pixel wide when it is only being used for outline.  Sometimes the color you've used to do lineart can be used as an additional color within your picture, but if the lineart gets too wide it can appear jagged in places that should be smooth and your piece will look really messy no matter how good a job you did in the coloring steps.  I know it's fun to submit art, but don't rush your work.  Be patient, compare your pixel placement and see what looks best before telling yourself it's good enough.  Truth be told, even the greats of pixel art would tell you that they could spend more time on their masterpieces.  

As far as deeper color, it's not necessarily needed in order to create form. It helps when adding depth, but form can be created from just a couple Hues distributed in the right spots. It's really up to you where those spots are, though, because it's your form and you have to decide how the light hits it, what texture might be created by the light, and how reflective the surface is.  

You're doing great in advancing with your art!  You're just getting to the part where it's about to get fun and a little complicated, so don't give up!   

Ok I made some deeper colour is this kinda what you were meaning?

This is really cool thank you I will use this as a base line, I agree with your points its a big help thank you 

Here's what I mean by defining a light source. It's not perfect and I know you would do it differently, but I thought it would be helpful to have a visual example to get you started.  Hopefully it shows that you can shade/highlight/and utilize your palette without sacrificing too much of your original piece.

Your lineart is always great! I've always had that as my strong suit because I love to sketch outside computer generated art.  But the thing I've had to learn, which I'm still working on, is that pixeling is less "drawing"as it is painting.  Your lineart is important, don't get me wrong, and you should keep using it to your advantage!  But your end result, despite either having an outline or keeping some original lineart as part of your developing style (or not if you prefer), should never rely on the lineart for distinguishing form.  The way to create form and depth in your presentation?  Use your palette(s) to give a clear direction of lighting.  In this piece, for instance, the lighting is somewhat erratic.  The lighting appears to come from the left, judging by his head.  Then the lighting comes from his lower left/viewer.  His arm suggests that it is coming from above, yet it's also somewhat unclear.  So I'd start there.  Give your subjects a clear light source.  I could be wrong and you actually intended more than one light source, but given the highlight colors you picked, I can only guess, because there is no hue shifting to indicate different colors of lighting, that there is only one light source.  

I don't want to overwhelm you so I'll leave it at the lighting.  Like I said before, I really dig what you're doing!  I hope you keep it up.  You can do it!