Might be they're just dirty and covered in mould of some kind, specially if they're in a shady spot. If the area is shady in the winter, or under trees, that can happen - you either need to get them up and clean them all, or replace with another material.

Do you mean raw wood chip, or treated bark chips? The latter do go black after a time. If you have a membrane, you could just top up with raw pure wood chip, which will whiten/bleach in the sun - call a local tree surgeon and see if they can give you a 6" depth delivery?

Avkg47, unless I'm mistaken (and it seems I'm not) it's quite clear from the question that Jamespaul is referring to white stone chippings - I've never seen white bark chips - new wood chips, yes, but not white chips.