Good question!!
1) White over white plastic: use the white primer
2) White and black over white plastic: use white primer
3) Red over blue plastic: depends...
Either primer will cover the blue enough that it won't interfere with the red. But, the shade of the primer itself will. The white primer will give a brighter red, and the gray will knock it down a couple shades. Colors on the warm side of the spectrum (reds and yellows) are susceptible to hue changes depending on the color they're sitting on. Darker and colder colors (blues and greens) are less susceptible. My MR2 was red plastic, and I used the gray primer and painted it blue, no problem. So...
3) Red over blue plastic: test both if you can, so see which shade will look better. If you have white primer left over, just use white. If there's no price difference, just use white. (Both kinds are the same price fo me...)
Another benefit of primers: finding and removing seam lines and imperfections. Give your bare model a light coat of primer all around, then give it a gentle sanding all over. You'll take some primer off the high spots and corners, but you'll also be able to see where seam lines are hiding, and where panel surfaces don't line up quite right. Then you can sand them down smooth, or at least note where they are, so you don't go through them while polishing...
