Edit in JSFiddle

var Button = React.createClass({
    getInitialState: function() {
        return {
            actived: false
        };
    },
    onClick: function() {
        this.setState({ actived: !this.state.actived });
    },
    render: function() {
        className = "button"
        className += this.state.actived ? ' actived' : '';
        return <div className={className} onClick={this.onClick}></div>;
    }
});
 
React.render(<Button/>, document.body);

<script src="http://fb.me/react-js-fiddle-integration.js"></script>

.button {
    width:  3rem;
    height: 3rem;
    background-color: #000;
}

.actived {
    background-color: #f00;
}