blockRainbow.js

document.write('<style>.block{width:30px;height:30px;position:relative;  float:left;display:block;}</style>');

function blockRainbow(gridId, sizeBlock, opacity) {
  var drawb = document.getElementById(gridId);

  var d;
  function createGrid(sizeBlock){
    for(d=1;d <= 16;d++){
      var innerDrawb = drawb.innerHTML;  
      drawb.innerHTML += '<span id="block-'+d+'" class="block"></span>';
      
      var rand_1 = Math.floor((Math.random() * 255) + 0);
      var rand_2 = Math.floor((Math.random() * 255) + 0);
      var rand_3 = Math.floor((Math.random() * 255) + 0);
    
      var blockNow = document.getElementById('block-'+d);
      blockNow.style.backgroundColor = 'rgba('+rand_1+','+rand_2+','+rand_3+','+opacity+')';
      blockNow.style.width = sizeBlock+'px';
      blockNow.style.height = sizeBlock+'px';
    }
  }

 
  createGrid(sizeBlock);

}

blockRainbow('draw-box', 50, 0.5);