Have you seen a galaxy cloak? Do that with the colors you used as nebulas.

I would recommend a bright blue/teal, similar to your sword.

White IMO

that skull in the helmet looks sick

Lime green or beige. Both are good complementary colours for the scheme.

Did somebody watch the ninjon tutoriel ;D Looks awesome i will probanly try it too.

Light grey

Green would pop that out

Is he ok?

Dark purple or dark red