The yellow on the teeth is tartar and will cause the gums to recede eventually and lead to dental problems. You might try one of those finger tip brushes that make it easier to reach in the back (if you have small hands). Giving raw uncooked knuckle or beef marrow bones also helps in keeping their teeth clean. Cooking the bones cause them to splinter so raw bones with supervision is the best (messy though).
You might also look into a product called "Life for Teeth" spray. It is basically a Grapefruit Seed Extract based product which is a natural antibiotic that kills bacteria (bad only). I don't brush and so far everyone's teeth are still perfectly white. I am also using it on a dog I recently got with very bad dark brown tartar buildup. It is definitely removing the tartar layer by layer.
http://www.tobysnaturalpets.com/petz.htm