Changing colors of lightning-button

Based on my understanding of the documentation, your CSS cannot reach into the child component of the lightning:button component: Styles defined in a component’s style sheet are scoped to the component. This rule allows a component to be reused in different contexts without losing its style. It also prevents a component’s styles from overriding styles in other parts of a page. The trailhead module Lightning Web Components for Aura Developers further expands on the concept. 

CSS Encapsulation with Shadow DOM Lightning web components use a web-standard mechanism called shadow DOM that hides the elements inside the component from the page that contains the component. Because lightning:button web components have a shadow DOM, styles defined in a component’s style sheet are scoped to the component. They don’t apply to parent, child, or sibling components. This rule is strict, but it allows a component to be reused in different contexts without losing its styling. It also prevents a component’s styles from overriding styles in other parts of a page.