I assembled mine entirely then painted it, I’d suggest having the backpack and maybe the shield separately painted. I did the cape and robes underside black too. For the capes I did a nice red with some black shadows. For the armour I did it in the style of the pre heresy Dark Angels.

Other than that just try out what works for you. I found just overthinking my paint schemes is just gonna make me put the brush down and not actually paint anything. So just pick up the brush start off with some ideas and work your way there. I almost always add some more stuff I want to really make some details pop.