The "L" in "18L" means that GM paint color code 18 was used on the lower portion of your car, the ground effects. From paintscratch.com you will find that GM paint code 18 is the same as WA7719, Medium Gray Metallic. You can order the matching paint from paintscratch.com.
Based on a search of duplicolor.com, it appears they no longer make the paint you need.
More info at http://fierosails.com/Paint.html
[This message has been edited by paulmckibben (edited 09-06-2017).]