Debra,
Look at the label. Most likely it is a mixture of a white plus some kind of yellow pigment. Most tubes marked Naples yellow are not the real thing.
The real thing is a beautiful lemony yellow, which used to be called, I believe, lead tin yellow and was used by Sargent among others. It is Lead Antimonate. Michael Harding (
www.michaelharding.co.uk) makes a beautiful one called Genuine Naples Yellow Light, that is useful to the end of the tube. It handles and mixes beautifully. It can be found here at
www.italianartstore.com.
It is pricey, a Genuine Naples Yellow always is.