A linear colour key, a the "Change Colour To" plugin... There's a hundred different ways to attempt this, but really it depends on your source material - in particular, how much white other than the car appears in the frame? In particular, silver and chrome surfaces on the car will often reflect light and look white. This complicates things greatly.
It's more than likely you will need to do some masking, or a lot of masking/rotoscoping, depending on the footage. If you can post a still from the footage we can give you much more educated advice.
The most efficient technique depends entirely on the shot. You'll need to create some kind of procedural matte so you can isolate the white car. Show us a screenshot, link to the video, or point to a similar image and we can give you some ideas. A shot like this is going to be fairly easy:
This one is going to require a bunch of rotoscoping by hand:
Changing a white car to red is almost certainly never going to look realistic. Other than that simply show us a frame from your clip. As the others have already said this will most definitely involve tons of work rather than just slapping on a desaturation effect.
It will be really tricky. All the reflections on glass, silver and chrome will need to be masked out - you'll need to fully rotoscope the white areas of the car. And then, as Mylenium says, it still won't look great because a glossy red car will also have white reflections, so you'll need to add white reflections back in to make it look realistic.