131 Silver / Pearl Light Gray
BrickLink: Pearl Light Gray
First used in (roughly) 1998, replaced in 2010 by 315 Silver Metallic.
This is a common color, and a source of much confusion on BrickLink. Parts in softer plastics that didn't show the pearl-ness much used to be considered Flat Silver on BL (even though they were Pearl Light Gray). "Flat Silver" is now used mostly for 315 Silver Metallic.
Lastly, to differentiate this color from Metallic Silver, note that Metallic Silver is a matte but shiny coating on top of the plastic, while Pearl Light Gray is the actual plastic color.