Err...
Well, I'd start with working out which version of the Ecto-1 you want to replicate with the paint. There's Ecto-1 as it appeared in 1983/4, there's Ecto-1 as it appeared after it's first restoration effort circa 2008... and then there's the car as it looked after the more recent second restoration.
If you wanted to paint it as the car is now, then it would be worth familiarising yourself with this gallery of photographs:
Link If you want to replicate the car as it appeared in 1983/4*, then use the DVD or bluray as your main basis for reference, in addition to this gallery of screen shots:
Link * It should be kept in mind that the 2005 DVD, and later bluray suffer from slight colour issues.