Ooh, good job on that first go! Nice selection of colours and having the patience to tackle all that armour is, I imagine, the main tool of a colourists trade!
I don't know if it's how they showed you on the course, but I'd suggest you've started with your colours too dark. I can see you're adding highlights after, but the best way to do it in my opinion is to go with a bright, mid-range set of colours, then go to layer >adjustment layer >hue/saturation and tint it darker (and cooler with a slight bluish tone). Fill the layer with black, then whatever you paint in white will be the darker shadow. Do another adjustment layer this time pushing up the lightness and saturation, and that'll be your highlights.
This explains it far better than me:
Lastly, your highlights are coming from a light source top left - they'd cast a shadow from the nose onto the left side (his left) of the face, so no highlight there!
Hope is ok being all picky, and that it's helpful rather than demoralising!