Depends on you wanting to follow the "fluff" of existing armies or something completely different.
I am partial to Iron Warriors but I like steel with black and yellow hazard stripe.
Some thoughts to help selection:
- Look at colour wheel and pick contrasting colours: orange/blue, yellow/violet, red/green, black/white.
- Decide if you want a cleaner look or dirty.
- This would then decide what metallics you want to use.
- Figure out what colour you want the base to be and try to keep it somewhat neutral to the colour scheme of the model.
- Black, white or grey prime? Will give a whole different look with each type.
- Tend to like to keep my weapons/guns the same colour for easy identification: typically I paint them red, I have been tempted with yellow for extra eye-gouging colour.
- Think of the skin colour for these guys: they will not be very normal in tone (unless that is what you want...).
These may hurt the eyes but certainly give ideas...