index.js

const React = require('react');
const ReactDOM = require('react-dom');
const PropTypes = require('prop-types')
require('./index.css');

const root = document.getElementById("root");

class ChangeColors extends React.Component {
  constructor() {
    super();
    this.state = { color: "black" };
  }
  handleClick() {
  this.setState(state => ({
    colorIs: !state.isToggleOn
  }));
}
  render() {
    const redButton = () => {
      this.setState({ color: "red" });
    };
    const blueButton = () => {
      this.setState({ color: "blue" });
    };
    const greenButton = () => {
      this.setState({ color: "green" });
    };
    const style = { color: this.state.color };
    return (
      <div>
        <h1 style={style}>Change My Colour!</h1>
        <p>
          <button onClick={redButton}>
            Red
          </button>
          <button onClick={blueButton}>
            Blue
          </button>
          <button onClick={greenButton}>
            Green
          </button>
        </p>
      </div>
    );
  }
}

ReactDOM.render(<ChangeColors />, root);