As
sloebertje noted above, the colors in
The City of Lost Children are "somehow exaggerated." The fantasy elements of the film are further emphasized by this dreamlike coloring of the city, the ocean (which is
green), the costumes, etc. It seems like it would be a lot of work to create scenery--and, well, water--that is so different and richly colored than normal, every day
stuff. However, it was achieved with very minimal effort on the part of the
scenery managers.
In order to make such wonderfully odd colors, they simply painted the actors white and adjusted the film's hue until they looked flesh-colored again. Neat trick, that, and a definite credit to whomever came up with such a technique: it works beautifully.