20 Nature / Milky White
BrickLink: Milky White
This color was first used in sets in 1963, for the nylon couplings shown here. It was commonly used for technic axles and antennae for a while.
*This color last appeared in sets in 2008, but because it's basically just uncolored ABS (or whatever plastic), Lego can use Nature at any time without having to "reintroduce" the color in the palette. It's certainly a current color in the LegoLand park model shops.