So one of my builds calls for light ghost grey. Vallejo has a matching color but it is a primer. Is there any reason I wouldn't want to use it as a final coat? I assume it would be fine.
It depends on how smooth the primer is when dry, I suppose. If it's fairly smooth, I would use it as the "actual" coat of paint, sure.
If you do want to paint over primer, BTW, it's best to use a colour of primer that's clearly different from the top coat. If you use light grey primer and then light grey paint, it'll be very hard to see whether you've got the top coat everywhere.
Would you happen to be referring to Vallejo Primer 70.615 USN Light Ghost Grey?
As far as I know, Vallejo created some primers to match the most common base paint layer colors on popular camouflage schemes as a convenience for the modeler.
Using a primer that is the same color as the first paint color of common camouflage schemes saves the time and effort of applying two layers: the first being the primer and the second being the first base color.
According to Vallejo, you can airbrush these primers like any other acrylic. https://acrylicosvallejo.com/en/product/hobby/primers-en/usn-light-ghost-grey-70615/