Make Div Partial Coloured

I know this question is asked before and also with good answers, however I still can't get this to work. What I try to do is make a div partial coloured. The problem I'm facing is

Solution 1:

Use a pseudo-element

* {
  margin: 0;
  padding: 0;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
}
.container {
  width: 50%;
  margin: auto;
  height: 100vh;
  border-left: 1px solid green;
  /* for reference */
  border-right: 1px solid green;
  /* for reference */
}
.mybar {
  height: 50px;
  background: lightblue;
  position: relative;
}
.mybar::after {
  content: '';
  position: absolute;
  top: 0;
  left: 50%;
  background: inherit;
  height: 100%;
  width: 50vw;
  z-index: -1
}

<div class="container">
  <div class="mybar"></div>
</div>

JSFiddle Demo