Just learning. I know it's all a bit of a mess. Comments and criticism/advice welcome.
6 Colors + transparency.
Edit: Taking Gennoveus' advice I've reworked some parts.
This version is a huge improvement.
Thanks for the advice. I hate the shape of the helmet so it's ok. I might try reworking bits of it.
Very nice; interesting character and like that you chose this angle for his body to be on, instead of straight ahead. I think the trouble with his hand is only that it is too small. Maybe you could try to make it larger and it could look better. The metal surfaces and his body are well done.
I think you could change the range of tones used for each material to give them a different feel. For example, use darker tones and avoid white for the (relatively) matte skin, white using more whites and lighter tones for the shiny metals, as an example. Just a thought; could be worth trying.
He has a bit of a sausage nose, and the majority of his face is one tone which makes it look slightly flat. If you try and break his face down into its basic blocky geometric shapes first, then soften it, you might get some hints on better shading. I also think you could be more aggressive with shadows.
Here's an example with my suggestions to just the head. I didn't do very well on the shiny parts and shape of the helmet but I hope it gets the message across.
Sorry I ruined the original shape of his helmet; here's a better example that preserves the shape slightly better i.imgur.com/v7bEXac.png