(PS: this suggestion is just my personal preference I should say..frankly I think everyone has their own 'ideal' set of colours)
try making the armor dark blue-ish violet / the hair yellow (faint yellow)
If you're planning to make one of the aspect of the painting to have a bright saturated colour/s, try putting it on the 'point of interest' ....and as for the other part of the painting make it less (not too less) saturated than the colour on the 'point of interest'..
from what I learned from looking at paintings similar to this, contrast in colour and colour saturation makes painting with simple subject to be really interesting and eye catching.
as for the light source...well it depends on what kind of 'feeling' you want your painting to have...if you want it to make it look 'menacing' I would suggest using :bright saturated red or blue green...if you just want it to be normal but dramatic just go for the basic Light source colour: faint yellow
here's some example of what i'm talking about:
Painting by Zoonoid @deviantart
(Bright blue green + low saturation red-orange)
forgot the artist's name on this one (Slightly low saturation red - red + bright saturated green)
Other set of colour suggestion: saturated red = hair, armor =black with some faint light blue/bluegreens on the highlights ( to make it interesting...the faint light blue/bluegreen is can be considered as the reflection of the sky)
gold = pole/spear and dark red = cape
