if and else if statements ignored, Javascript

I have a purple button. It becomes green onclick, even if in the if statement instead of purple I wrote Yellow. Then it remains green, although the else if statement should make it purple again onclick. What's wrong with my code?

`var m = document.getElementById ("buttonanimation");
    function changecolor(){
        if (m.style.backgroundColor="yellow"){
        m.style.backgroundColor="green";
        } else if (m.style.backgroundColor="green"){
        m.style.backgroundColor="purple";
        }
    }`