Just mix all the cans together and you'll have exactly the color you want.
He is about right. We were trying to find a nice shade of aqua blue for the girls' room. Bought some sample jars, didn't like any of them, added white to one to get what we wanted and had them match that.
When we did the (at the time) nursery, I wanted a light apple green. We had samples, painted on the wall and chose. Hubby got a full gallon of the paint, painted three walls before he came to the samples on the wall, and we noticed the color was way off. It was more of a Margarita lime green. Had to run back to Ankeny HD to get the right color. Part of why we have only painted 2 rooms in 9 years.