I have tried to change the border color of select 2 version 4 with when it is opened:
.select2-dropdown--above {
border: 1px solid blue !important;;
border-bottom: none !important;
}
.select2-dropdown--below{
border: 1px solid blue !important;;
border-top: none !important;
}
Code at: http://jsfiddle.net/7c0Lm033/
Here is what I get:
As you can see I can not change the color of select 2 head color. I could not find any unique selector which can change the marked color border.