The gutter (spacing between columns) is created with padding, not margins. When you adjust the margins it throws off the grid. You can use inner DIV's...
<div class="container-fluid px-0">
<div class="row no-gutters">
<div class="col-6 text-light text-center">
<div class="bg-dark mr-1">Hello</div>
</div>
<div class="col-6 text-light text-center">
<div class="bg-danger">Hello</div>
</div>
</div>
</div>
or, force the row
no to wrap with flex-nowrap
...
<div class="container-fluid">
<div class="row flex-nowrap">
<div class="col-6 bg-dark text-light text-center mr-1"> Hello </div>
<div class="col-6 bg-danger text-light text-center"> Hello </div>
</div>
</div>
or, simply use the padding utils to adjust the column spacing.
https://codeply.com/go/p4NpmmRxmb
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