When you are shootng with dark people in the shot overexpose by .5 of an fstop. In the one with the motorcycle the white on the bus looks grey because that is how your meter views it. A half stop increase in that shot would have made the bus white and added detail to the cyclists.

Another approach often used is switch to spot meter, zoom in, take a reading off any face, arm etc. and set your meter accordingly for all your people shots on that particular outing.

Tegan