Take a white sheet, crumple it up and throw it on the bed. Depending on whether it's a warm or cool white (and the lighting) you will notice that only the very elevated areas under direct light actually appear true white. The rest will either be shades of grey or beige (again, depending on the type of white and lighting).
IMO, go cool white for armor. Use a very light grey (I am not an exclusive GW paint user, so I won't drop paint names) as your base coat, shade with a slightly darker shade of grey. I would opt for multiple coats over a single coat of a darker colour for this. Do a highlight of white + your basecoat, and extreme highlight of pure white.
On contrast, for warm whites (tabards etc) replace grey with beige.
Experiment a little to get the look you are going for. Remember not to use pure white as your basecoat, if you do, you have no options for highlighting.
Cheers!
