In PHP and pretty much any programming, there's another element to the IF statement. That is, the ELSE statement.
In an IF statement, you give it a condition. If the condition evaluates to true, then the corresponding code is executed. If the condition evaluates to false, then the ELSE code block is executed. Note the ELSE is not required, as sometimes it may be appropriate to simply do nothing.
In your case, you'd want to write something like this:
<a href="http://www.mywebsiteaddress.com/specials"<?php if( $thisPage == "Specials") { echo ' class="active"'; } else {echo 'class="yellow"';} ?> title="Summer Specials">Summer Specials</a>
So if the condition of the IF statement evaluates to true (if $thisPage is equal to "Specials"), give the anchor tag a class of "active", otherwise (if it evaluates to false) give it the class of "yellow".