Hi Pedro,
in my opinion, you wanted too much too fast, your illustration is very ambitious, good you tried it

I think your drawing lacks in the fundamentals and therefore no kind of tweaking will make it shine,
id redo it from ground up, several points:
- the pose is a bit weak and i'd show arms or hands, draw a lot of them, get reference
- get the drawing right, there are several issues with anatomy and general "3D-ness" some of your shapes are "flat" like the eyes or the hair
- if you think you got a decent drawing of the figure (no armor involved yet), post it here, flip it, redraw, study parts where you got problems
- if you got your basic figure, think about the armoursuit, keep it simple, your illustration shows that you are not sure about the forms (at least i can't see them clearly)
- get reference
- if you got a decent drawing THEN you should begin to paint.
- Your painting suffers from the weaknesses in your drawing so: The value should describe the form, depending on the lighting,
- Establish a good value base (darks and lights and midtones) keep it simple and aim for a strong statement, keep a simple lighting scheme, you seem to have some kind of two point lighting scheme in mind. Your painting has not a clearly established value-range. The colors on top are way too saturated.
- stay away from color until the values are correct. Color is the last thing you want to care about, if you got a decent painting with strong values, getting color in is a lot easier.
- overall: Spend A LOT OF TIME...i guess for some kind of polished illustration like this you would need at least 15 hours or so.
You said you did this a a sort of practice: I think you should tackle single and simpler problems first: Valuestudies of simple objects, figures and poses, faces, features and so on. I hope i didn't offend you, keep it going, draw a lot....rinse repeat.
Anyhow, keep it going!