They are black and white because then you have to only paint one half. The tarmac is already black, you just have to 'paint' on the white stripes. According to the Specification for Highway Works (Chapter 3), the stripe against a kerb must always be black. The white sections are either put on using a type of spray white (as used nowadays for quickness in white lining operations), or more traditionally thermoplastic screed. This is what is in the big cauldrons on the back of white lining trucks. It is usually laid about 3mm thick and lasts for several years (whereas the spray doesn't). Immediately after the screed is laid (ie still hot) ballotini is shaken on to it. This is a very fine glass 'dust' in the form of minute balls, which reflect headlights in darkness. This is also used on normal white lines. Experiments were made with using large white tiles for zebra crossings (about 2' square I think), but these have largely been replaced, as if one gets dislodged, it makes the crossing unsafe for pedestrians.