How to change default dropdown color for bootstrap 4 selectpicker?

My form is consists of single and multiple dropdown select. So I prefer to use 1 class only for my single and multiple which is selectpicker. However, I want to change the default grey color for Bootstrap 4 Dropdown selectpicker class to clear color. Where is exactly color that needs to be changed in CSS file for .bootstrap-select class?

Current View (actually this no disabled but the view is like disabled)

To be like this (I try to use custom-select2 class as example of clear color)

HTML

<div class="form-group">
    <select id="region" class="font-14 selectpicker form-control">
        <option value="">-- REGION --</option>
        <option value="allregions" id="allregions">All Regions</option>
    </select>
</div>