var onColor = "green";
var offColor = "blue";

function imgOn(id) {
  swap(id, offColor, onColor);
}

function imgOff(id) {
  swap(id, onColor, offColor);
}

function swap(id, oldColor, newColor) {
  var img = document.getElementById(id);
  var regexp = new RegExp("-"+oldColor+"\\.png");
  var replace = "-"+newColor+".png";
  img.src = img.src.replace(regexp, replace);
}