Kirk is right.
Single coil pickups have a plastic cover over the bobbins and windings. This cover is easily replaced and can be any colour, because it's just an injection-molded piece of plastic that goes over the guts of the pickup.
Humbuckers are wound right on the plastic bobbins. The windings are then wrapped in tape to protect them, and on the original Gibson humbuckers, a nickel cover was then attached. Nowadays, most of the humbuckers we see don't have the nickel cover, so the bobbins themselves are exposed. The exposed part you see isn't a "cover"... it's the guts of the pickup. Thus, the pickup manufacturers have started making them in different colours.
There's no way to change the colour of your humbuckers other than replacing them, painting them or adding cosmetic pickup covers like Roger at Jeannie makes.