Assign colors

HTML

<script src="https://code.jquery.com/jquery-2.1.4.js"></script>
<div id="Content">
  <p>
    <input class="btn_colors" data-color="#007FFF" type="button" name="blue" id="blue" value="Blue" />
    <input class="btn_colors" data-color="#F2B400" type="button" name="yellow" id="yellow" value="Yellow" />
    <input class="btn_colors" data-color="#66B447" type="button" name="green" id="green" value="Green" />
  </p>
  <div class="column">
    <div>20</div>
    <div>60</div>
  </div>
  <div class="column">
    <div>72</div>
    <div>71</div>
  </div>
  <div class="column">
    <div>88</div>
    <div>87</div>
  </div>
  <div class="column">
    <div>64</div>
    <div>53</div>
  </div>
  <div class="column">
    <div>90</div>
    <div>79</div>
  </div>
  <div class="column">
    <div>54</div>
    <div>73</div>
  </div>
  <div class="column">
    <div>74</div>
    <div>63</div>
  </div>
  <div class="column">
    <div>98</div>
    <div>57</div>
  </div>
  <div class="column">
    <div>74</div>
    <div>63</div>
  </div>
</div>

CSS

.column {
  float: left;
}

.column div {
  border: 1px solid #000;
  padding: 4px;
  margin: 2px;
  width: 15px;
  height: 15px;
  text-align: center;
  cursor: pointer;
}

JavaScript

// variables
var buttons = document.getElementsByClassName('btn_colors');
var numbers = document.querySelectorAll('.column > div');
var current_color = document.getElementById('green').getAttribute('data-color');

// listener for button clicks
for (let i = 0, c = buttons.length; i < c; i++)
  buttons[i].addEventListener('click', set_color, {
    passive: false
  });

// listener for number cells
for (let i = 0, c = numbers.length; i < c; i++)
  numbers[i].addEventListener('click', set_bg, {
    passive: false
  });

// functions
function set_color(event) {
  event.preventDefault();
  current_color = this.getAttribute('data-color');
}

function set_bg(event) {
	if(this.classList.contains('clicked'))
  {
  	this.classList.remove('clicked');
    this.style.backgroundColor = 'transparent';
    return ;
  }

  this.style.backgroundColor = current_color;
  this.classList.add('clicked');
}