Hi Traividya ,
You can try to do it using CSS to highlight the first n children, Give your reference selector a class in the appearance tab , example test.
Then add this style in your custom CSS.
.test option:nth-child(-n+5)
{
background-color: #c3ebd9;
}
Result
for the elements selected here You can refer to this link
Styling options is something that has restriction from the OS itself , it might not be what you wanted exactly but i hope it would be helpful.
Thanks,
Sufian.