What was the shortest-lived color?
Further to Alex's answer, from that source I found eleven colours that only lasted a year:

Fabuland Orange 1983
Very Light Orange 2000
Clikits Lavender 2005
Chrome Black 2009
Chrome Green 1999
Satin White 2020
Satin Trans-Light Blue 2020
Satin Trans-Dark Pink 2020
Glitter Trns-Orange 2020
Speckle Black-Copper 2006
Speckle DBGray-Silver 2006

Of course the four 2020 colours may continue next year!
Fabuland Orange was only used for one item in one set for one year, so could be considered the rarest colour.  The set was &quot;Peter Panda Takes a Bath&quot;, and the part was Peter Panda's tub, a part barrel affair.