What’s the problem? The email reader overrides the link colour and styling set in your email code, instead using CSS to show text links in their own default font-size and shade of blue. These links become hard to read if they appear on a dark background colour.

What to do Text formatting set in the HTML hyperlink tag immediately surrounding a text link will override all other formatting.

Often in email clients you will see that Email Clients will add in there own styling as you will see this render as User Agent Stylesheet. Make sure to inline styles for this link tag directly on the link tag such as the following code. 

<a href="#" style="color: #fff;">Click Here</a>