The Eiffel Tower is repainted every seven years. Contrary to what you might expect, it is not painted uniform black, but rather is graded from light grey (at the top) to dark grey(at the bottom)

The reason for this is, due to the height of the tower and the angle at which you normally view it, coupled with the dimming effects of looking through the grim pollution of Paris, this is the only way to make it look like it has a constant colour. The level of pollution is recalculated every time it is redone.

I learnt this when hitchhiking in Germany; one of my lifts was a man who worked for the industrial paint company that was contracted to paint it last time.