for 800nm to 400nm, we need gamma=2. So v~.86c. At 20m this gives you ~80ns.

The sticker should also say something like "But don't worry, we're going to be evaporated in a huge explosion anyway, due to the gigantic release of energy when you crash into my car in a few nanoseconds."

does the observer have to be able to see the sticker with normal human eyesight? if so that constrains distance (which maybe doesn't matter?)

I want to know how fast you need to go to be color shifted on a traffic camera. The more I think about it the less likely getting enough light to be picked up by the camera might be an issue even with perfect placement