JSFiddle - React, Tailwind, and code Playground

HTML

<img width="100" height="100" src="http://a/" />
<img width="100" height="100" src="http://b/"/>

CSS

img {
  background: black;
}

JavaScript

var x;
var allimgs = document.getElementsByTagName("img");
var imglen = allimgs.length;
var getColorFromUrl = function (src, callback, index) {
  if (src === 'http://a/') {
  	callback([256, 0, 0], index);
    return;
  }
  callback([0, 256, 0], index);
}
var change = function(color, index){
  console.log(JSON.stringify(color))
  allimgs[index].style.background = 'rgb('+color[0]+','+color[1]+','+color[2]+')';
}
var colormix = function(){
  for(x = 0; x<imglen; x++){
      getColorFromUrl(allimgs[x].src, change, x);
  }
}

colormix();