Enemeink is correct. Sort of. First, my apologies if I might be misunderstanding your post, enemeink.
The Millenium Yellow is a tint coat, and an optional paint color you paid extra for ($795). In the info I have (somewhere around here! LOL) Its called Millenium Yellow Tintcoat, to be exact. I can't really say for sure for the Velocity Yellow if thats a tintcoat or not. I suspect it is.
Also, enemeink is correct about the automotive paint shop. If you manage to go there with a GM paint code, they'll mix it right up for you.
Good luck and keep us posted. I'd like to see the progress on your kits. I have a Revell snap kit C6 that I want to do up like my 1:1. Maybe it'll give the motivation to start on it!! LOL!!