Original: Sometimes the aim is to simulate a color that is perceived by the consumer as natural, such as adding red coloring to glacé cherries (which would otherwise be beige), but sometimes it is for effect, like the green ketchup that Heinz launched in 2000.
Simplified: An example for this would be adding red coloring to glacé cherries (which would otherwise be beige). Sometimes food coloring is done for effect,  like the green ketchup that Heinz launched in 2000.