am assuming custom_class to be your class to override heading color of the panel. just add !important to it or add inline styles or a id and add styles as they will have more specificity over class added styles.
with !important
.custom_class{ background-color: red !important; }
with !important
with ID :
#custom_id{ background-color: red; }
-with inline styles :
<div id="custom_id" class="panel panel-default">
<div class="panel-heading custom_class" style="background-color:red">
</div>
</div>