Here's what I mean. He had two small dents and one corner of the front bumper needed to be repainted. I told him it would be fairly expensive, and the biggest problem would be matching the color. His car is six years old, and the paint is faded, even though it has always been kept in a garage and cleaned and wax properly. The paint is still faded, and matching the color so that it doesn't look like it has been painted in spots will be a big challenge.
Many car manufactures use metallic or irridescent colors that fade over time.
And I've even seen 3 different paint formulas for the same color. Another factor in getting the color right is how many coats of paint are applied and how may coats of clear are applied. Now that you're probably totally confused, there are a few more things that need to be taken into account as well to get the proper color. Humidity needs to be considered to select the proper thinner for the paint. And if that's not enough, some colors have very little pigment in them, and the final color you see will depend on the color of the primer used under it.