Originally Posted by JGreenwood07

The reason the shades are different is that the bumpers, side skirts, etc are are plastic, while the hood, doors, etc are metal. The porousness of the plastic absorbs the base coat and primer differently than the metal. This causes the paint to settle and refract light slightly differently and gives the illusion of the colors being slightly off.

Also, as others pointed out, sometimes the plastic panels of the car are painted separately from the metal ones at the factory and can have very slight variances in paint color mixtures.