JAVASCRIPT

set property dom javascrpt

document.getElementsByTagName("H1")[0].setAttribute("class", "democlass");

set property js

There might not seem to be a difference with 'color' but consider:

element.style.backgroundColor = 'blue' // works
element.style['backgroundColor'] = 'blue' // works
element.style['background-color'] = 'blue' // does not work

element.style.setProperty('background-color','blue') // works
element.style.setProperty('backgroundColor','blue') // does not work