Why are plane vapour trails white not black?

I believe it's a combination of fuel exhaust and rapid compression of the moisture in the atmosphere due to the speed of the plane.

ANSWER:It is from the hot exaust from the airplanes engines hitting the low pressure cold air in the upper atmosphere. It causes condensation and makes a cirrus cloud in a long line behind the plane. AnswerWhat is happening is that a combustion byproduct (either of a jet or piston engine), water, is condensing. Maybe even freezing. After all, atmospheric conditions up high usually include very cold air. When conditions are right, the condensate trail can be seen forming a short distance behind the plane. That's not due to compression.

When we "see our breath" on a cold day, we are seeing the water vapor in our breath cooling and condensing.