I've gotta agree with Zoom Zoom. You need to seal the plastic, as even with grey primer, some white primers will still turn pink if you didn't seal the plastic...
Tamiya grey primer, then white doesn't seem to turn pink, but many automotive plastic primers, even if the grey is good and even, it bleeds...
If your top coat is to be red, it really doesn't matter.. Or black or silver for that matter but for white, its critical to stop the bleed..
I hate red plastic for this very reason.... It might be good for the builder who doesn't paint their models, IE kids etc, but for those who do, its a right pain in the butt..