Javascript if statment issue [closed]

i Have the following working

function changeCellcolor(cellID)
{
var cel=document.getElementById(cellID);
//if (cel.style.backgroundColor='#DDDDDD'){ return}
if (cel.style.backgroundColor=='')
  {
  //cel.style.backgroundColor='#DDDDDD';
  cel.style.backgroundColor='green';
  }
else
  {
  cel.style.backgroundColor='';
  }

}

However I am trying to skip the function when the cell background is a different color

I tried this...

function changeCellcolor(cellID) {
  //alert (cellID);  
var cel=document.getElementById(cellID);
  if (cel.style.backgroundColor='yellow') {          
  } else if (cel.style.backgroundColor=='') {
    cel.style.backgroundColor='green';
  } else {
      cel.style.backgroundColor='';
  }

}

The problem is this always runs what is in the first If statement. It turns the cell background to yellow