I used the below CSS style to change the background color and text color of a calendar overlay.
<style type="text/css">
.ms-acal-color9{
background-color:Yellow !important;
}
.ms-acal-color9 a{color:#000!important;}
</style>
<style type="text/css">
.ms-acal-apanel-color9{
background-color:Yellow !important;
}
.ms-acal-apanel-color9 a span{color:#000!important;}
</style>
While the background color and text color change is working in the calendar view. But in left hand navigation, only background color change is working, text color change is not working.