Bootstrap 4, how to make a col have a height of 100%?

Use the Bootstrap 4 h-100 class for height:100%;

<div class="container-fluid h-100">
  <div class="row justify-content-center h-100">
    <div class="col-4 hidden-md-down" id="yellow">
      XXXX
    </div>
    <div class="col-10 col-sm-10 col-md-10 col-lg-8 col-xl-8">
      Form Goes Here
    </div>
  </div>
</div>

https://www.codeply.com/go/zxd6oN1yWp

You'll also need ensure any parent(s) are also 100% height (or have a defined height)...

html,body {
  height: 100%;
}

Note: 100% height is not the same as "remaining" height.

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Related: Bootstrap 4: How to make the row stretch remaining height?

Use bootstrap class vh-100 for exp:

<div class="vh-100">

I have tried over a half-dozen solutions suggested on Stack Overflow, and the only thing that worked for me was this:

<div class="row" style="display: flex; flex-wrap: wrap">
    <div class="col-md-6">
        Column A
    </div>
    <div class="col-md-6">
        Column B
    </div>
</div>

I got the solution from https://codepen.io/ondrejsvestka/pen/gWPpPo

Note that it seems to affect the column margins. I had to apply adjustments to those.