toRGBA = colorObj => {
	const c = objectAssign({ a: 1 }, colorObj); // rgb-to-rgba:  alpha is optionally set to 1
	return `rgba(${c.r}, ${c.g}, ${c.b}, ${c.a})`;
}
Transform color object to rgb/a
@param  {Object} colorObj RGB(A) color object
@return {String}       	 rgba css string