Californian squirrels are called Western grey (Americans would say 'gray') squirrels. As you can see in my picture they exhibit a form of coloration known as counter shading. The dorsal fur is a silver gunmetal gray, with pure white on the underside; there may be black flecks in the tail. Ears are large but without tufts. The ears turn reddish-brown at the back in the winter. The tail is long and typically very bushy. Tree squirrels undergo a complete head-to-tail molt in the spring and a rump-to-head molt in the fall. Tail hair is replaced only in the spring.
They are apparently far more shy than the other two types of squirrels. Well... I was personally feeding red squirrels in Poland and they were not afraid to come close to me and have their nuts. And squirrels at Jephson Gardens in Leamington Spa were literally eating nuts straight from my hand. In contrast Western grey squirrels will generally run up a tree and give a hoarse chirping call when disturbed. Well... that's what a squirrel in LA did when I was trying to approach her :-)