to paint over a dark color, would you first use a primer...would you color the primer and make it grey?

I went to the paint store and was told that if I let her put grey color in the primer that it would cover better than if it was left white...........

I found it to be not a great idea and I wish I had told her...no...but she is the pro so I thought she would know what she was talking about. Is she right? thanks Gayle