Sorry completely mis-read the question – maybe coffee time….
So the Update Cart button inherits the standard button colors. However, when it is disabled ie. no update has been made, its opacity is reduced.
So to change just that button you would need this CSS:
/*Static */
.woocommerce .cart .button[name="update_cart"] {
background-color: #F00;
color: #fff;
}
/* Hover */
.woocommerce .cart .button[name="update_cart"]:hover {
background-color: #0F0;
color: #fff;
}