Colour difference
You can use colour differences to find out if a car has had major damages. You have to keep in mind, however, that resprayed body panels can hide as little as paint damage. So, it is purely an indication for possible restored collision damage. Colour difference is only a motivation to do a thorough check. You use the resprayed body panels as a starting point for your search for structural damage. Take a special interest for the chassis, suspension and the correct mounting of the body panels.
Do not think that it is easy to spot a colour difference. It is easier to spot indicators that give away resprayed panels. These indicators are now described.
Door seals
When a car gets resprayed the parts that are not meant to be painted are covered up. To keep the costs down, parts like the front and rear windscreen and the rubber door seals are not removed. Because of this saving, you can find out if the car is resprayed. Look close along the sealings and try to peek underneath them, if the original paint is showing itself from under the newly sprayed paint.
The photo below shows a cars door. This door has been repaired awfully after a minor damage. The raggedy edge is a couple of millimetres wide and shows clearly how bad of a job the painter can do when taping the work area before painting. Although the car is generally in a good state, the badly painted door looks just awful and scares future buyers of.