You're going to have to use conditional and/or vml statements if you really want something specific to happen with outlook which is not happening with the default code.
I've seen a couple of fixes for this type of issue.
Try adding a table around the link text
<table><tr><td style="color: #FFFFFF">Learn more</td></tr></table>
Or try including the
<font color:#ffffff>Learn more</font>
Hope one of those work for you.