For content blocks, you would see an options toolbar like this, you would be able to change text color.

The change in color doesn't show up in Preview though. The other option would be to duplicate 'Link to Mirror Page' content block, make sure 'Depends on Format' is selected and add color on a tag as below
<% if ( document.mode != 'mirror' && document.mode != 'forward' ) { %><p align=center>
<font style="font-family: Verdana, Arial; font-size: 10px">
If you're having trouble reading this message,
<a style="color: #000000;" href="<%@ include fragment='MirrorPageUrl' %>" _label="Mirror page" _type="mirrorPage">click here</a>
</font>
</p><% } %>
Hope this helps.