Hi sootie, I would buy two bales of towels, one plain white & one plain black. You can either put alternating black & white towels out or stick to one or the other colour, thats what I did anyway with a similar colour scheme.
I agree with skyep, I have got a black and white bathroom as well and I have black and white towels. I generally have either 2 white or 2 black + 1 of the other colour out.
Any colour at all will go with black and white. Just as any painting or picture will be OK on a white wall, think of the towels as a decorative feature.
Cream should never be put with black and white, bold colours should be used but in moderation. Red is an excellent choice for towels, bath mats candles etc. But if red isn't your thing most bold colours will be fine, but not neutrals.
I'm with droopdrawers on this one - I've got the same colour tiles and have gone for black and cerise towels (from Asda). The cerise makes it look really up to date - and not like everyone else's boring neutral or blue bathrooms!