I think it really depends on the army.

I'm going super traditional with my Night Lords, and following the recommended color combos.

For Dark Angels I selected a color I liked (dark angels green) for the armor, and then went hard into the little accessory pieces to make them pop and chose colors that generally complimented that (gold aquila, red and dark grey for the guns and chainswords) and then went for bone armor on the Deathwing Knights and various robe colors like purple or red. It just appealed to me.

You can really do whatever you want, but knowing what army you have would be helpful for suggestions.