border-right-color

border-right-color: red;

border-right-color: #32a1ce;

border-right-color: rgb(170, 50, 220, 0.6);

border-right-color: hsl(60, 90%, 50%, 0.8);

border-right-color: transparent;

<section class="default-example" id="default-example">
  <div class="transition-all" id="example-element">
    This is a box with a border around it.
  </div>
</section>

#example-element {
  background-color: #eee;
  color: #000;
  border: 0.75em solid;
  padding: 0.75em;
  width: 80%;
  height: 100px;
}

/* <color> values */
border-right-color: red;
border-right-color: #ffbb00;
border-right-color: rgb(255, 0, 0);
border-right-color: hsla(100%, 50%, 25%, 0.75);
border-right-color: currentColor;
border-right-color: transparent;

/* Global values */
border-right-color: inherit;
border-right-color: initial;
border-right-color: unset;

<div class="mybox">
  <p>
    This is a box with a border around it. Note which side of the box is
    <span class="redtext">red</span>.
  </p>
</div>

.mybox {
  border: solid 0.3em gold;
  border-right-color: red;
  width: auto;
}

.redtext {
  color: red;
}