You tagged your post with "Sass". Are you using the Sass/SCSS preprocessor? If so, you'd declare and use a variable like this:
$myColor: red;
.sourceClass { color: $myColor; }
.destinationClass { border-color: $myColor; }
If you're not using Sass, you can read about native CSS variables - which are currently working in Firefox and Chrome, but not IE/Edge.
Lastly, there is a possible solution supported in all current browsers, which would be applicable depending on your DOM hierarchy: currentColor.
If your .destinationClass
is a child of .sourceClass
, and therefore is inheriting color: red
, you could simply use border-color: currentColor
to take that color and use it as the border color.
Hope this helps!