three.js/examples/jsm/renderers/common/Color4.js

import { Color } from 'three';

class Color4 extends Color {

	constructor( r, g, b, a = 1 ) {

		super( r, g, b );

		this.a = a;

	}

	set( r, g, b, a = 1 ) {

		this.a = a;

		return super.set( r, g, b );

	}

	copy( color ) {

		if ( color.a !== undefined ) this.a = color.a;

		return super.copy( color );

	}

	clone() {

		return new this.constructor( this.r, this.g, this.b, this.a );

	}

}

export default Color4;