Primer or regular flat gray acrylic over dark green plastic?

Roger that!  Tamiya fine scale white primer in a spray can will cover the green color nicely in one coat, and it is thin enough to not obscure surface details, and will hide minor problems like scratches.  Good stuff.

My two bits.

Priming is more a process than a coat. There's going to be a little filling, sanding and repriming to do.

Besides solving any adherence issues, primer can be easier to use from a spray can and not having to do airbrush set-ups until ready for the final coat.

I'd go flat white with the primer. That way you can better judge coverage of the later gray finish. That would be hard to do over gray primer.

LDF, It all depends on how much grief you're prepared to deal with later. If you have a high tolerance for imperfections, then just spray your desired color on and move on. If on the other hand, you would rather not risk having scratches in your paint or spots where the paint just peeled off that allow the dark green color to show through, then you need to properly prepare the surface for painting. You actually could have done all that in nearly the time it took you to get online and ask your question. A good primer will go on so thinly that you will not lose any detail. You just need enough coverage to give your top coat something to stick to. I recommend wiping the plastic surface down with some isopropyl alcohol to remove any lingering mold release agent and then spraying on a primer. The Tamiya spray can stuff works great. Just spray in light layers till you just barely have it covered. Some people really like Vallejo primer, but I prefer something with more bite. The best in my humble opinion is Mr. Hobby primer. If you'll take those extra 30 minutes maximum, you will likely save yourself a lot of grief later. Barrett Duke

The main purposes of primer are to improve adhesion, and to cover small dents and scratches. If adhesion is not a problem and the model is everywhere smooth, then almost any paint will cover the plastic color well.  The best color covering is from metallic paints like aluminum, silver, or gold, but unfortunately some of these do not provide the best adhesion for following coats.

Acrylics tend to scratch very easily. So for painting a light color acrylic over a dark color plastic, a good gray Primer base would be advisable. And it gives the added benefit if showing any flaws you may have missed.

i personaly would use primer first. a good primer, eg vallejo's, won't obscure any details and it will make for a better finish. you wont see the green coming through.