45 Light Blue / Light Blue
BrickLink: Light Blue
This color was never used very much since its introduction in 1995 as Aquazone baseplates. Belville used it more than any other theme. Last used in 2003-2004, I believe, after which it was replaced by 212 Light Royal Blue (Bright Light Blue).
Important note: This color is frequently confused with Medium Blue, a very common color in modern sets. Anyone selling on BrickLink should think long and hard about adding any Light Blue parts, because odds are it's actually Medium Blue :)