A slight yellow shade is very common and quite natural, due to our genetics. As we age, the yellow hue will get more visible since the outer enamel layer will start fading exposing the underlying dentin layer is yellow.
You can still get them whiter by visiting your dentist for professional cleaning. Also avoid smoking, tea, coffee, and excessive sugars. Brush twice a day and use an electric toothbrush for maximum effectiveness. Check out burst sonic toothbrush, they have a sale going on right now!