Some companies release kits in red so its easier to paint the kit red.
Or yellow so you can paint it yellow.
Red and yellow are hard to get good coverage on a kit without using 4 or 5 coats of paint
So if the plastic is already red you only need 1 or 2 coats. No primer..
But if you want it a different color you will probably use 4 or 5 coats anyway just to try and hide the red.
I prefer kits in white. Its just easier that way.