It's actually very possible to see a preproduction car cruising around the streets. Like to get a real world feel for it. There are some things you just can't simulate very well in test tracks and such. The new (preproduction of course)Camaro is currently bounding around parts of Australia doing some of this very same testing. It's been spied by a few automotive spycams. People in Ohio and Michigan may be familiar with this. I saw a preproduction Solistice and a preproduction Caddy STS a full year before release. They give them to higher ups, like plant managers, to test drive and evaluate. And even to park in their driveway, as it's good free publicity.
Another is to gauge public reaction to a vehicle. Sometimes you'll see a really sharp car, with no manufacturer identitifiers on it, but people will stop and say "hey, great car, what is it!?". It's kind of a free focus group/public reaction.
A lot of times, you'll see cars in camo instead of just being there for everyone to see. They'll hide certain body lines and things.
Like this:

As far as the colors, the new Camaro is inspired by the 1st Gen Camaro, particularly 68-69's. And many of those had racing strips and such. The convertible Camaro concept, was actually painted to match a popular model sold that had black stripes and red sidewall tires.

Slap some yellow on him and it looks mighty close to BB. The rest of the explanation is just your own suspension of disbelief, aka you'll accept it because you want to, like chosing to watch a movie about 30 foot tall robots doing battle on Earth.