Hi Mike.

Thank you for your purchase.

The CSS needs to target the "a" parameter of a hyperlink (<a href...), so the CSS should be:

table.wpDataTable td.hyperlink_button > a {
color: white !important;
}

Also, that will remove the "underline" decoration, so to include it back, you should add another line:

table.wpDataTable td.hyperlink_button > a:hover {
text-decoration: underline !important;
}

Hi again Mike

Since this is a button, you need to add the class in the Data tab in column settings, under "Button Class":

If that doesn't work either, please provide me a temporary WP-admin login for your site where this happens, so we could log in and take a look ‘from the inside’ as that’s the most efficient way to see and resolve the issue. We do not interfere with any data or anything else except for the plugin (in case that’s a production version of the site), and of course we do not provide login data to third party. 

You can write credentials here just check Private Reply so nobody can see them except us.