Farmboy is right - you need to take the car to a paint store for a color match. All colors fade and yellow fades so slightly you may not even notice that it has. Plus, it is going to make a difference brand to brand on what the formula is. Most auto paint suppliers can look the factory code up and then mix it in whatever brand you choose but depending on how long ago your car was painted, it still may not match. Furthermore, final color will also depend on your primer used on the rest of the car - yellow is pretty opaque and is affected by the layers underneath it.