When you customize your links in the styles tab, you'll be changing the link color for all links in your email. If you want to change the link color for only one link, you'll need to use inline html styling with the !important tag. This is required to force the link color to be different than what's set in styles. Your code should look something like the following.
<a href="URL" style="color:#HEXCODE !important;">Text to hyperlink</a>