soulite: I wouldn't get my hopes too high, full coloured manga is quite uncommon, atleast outside of doujins.... Most doujins are also black and white too, but I feel like there are quite a few full-coloured doujinshis.
It's obvious that most manga is going to be in black and white, due to the fact that it's so much cheaper to produce. Not only do they not have to decide what colour each thing is going to have, they will also save on production costs, staff costs, and so on. Most manga has a coloured front on them, and that mostly gives an idea of what the author thought that character X would have as a haircolour. But to be honest, if we were to bet on what hair colour they have, they'd probably have black hair, just like everyone else in Japan (the real world).
I don't really know what I prefer, I don't really care too much I guess... I suppose it may be annoyign with full colour though, since you may be so used to black and white, that instead of feeling special, it feels weird and annoying to have full colour.