select2 .select2-container--open .select2-dropdown--below code example

Example 1: select2 .select2-results .select2-highlighted

#menuOption1-container  .select2-results__option--highlighted {
    background-color: #50831F !important;
}

#menuOption2-container  .select2-results__option--highlighted {
   background-color: #28915F !important;
}

Example 2: select2 .select2-results .select2-highlighted

$(document).ready(function() {
    $("#menuOption1").select2({dropdownParent: "#menuOption1-container"});
    $("#menuOption2").select2({dropdownParent: "#menuOption2-container"});
});

Example 3: select2 .select2-results .select2-highlighted

<span id="menuOption1-container">
    <select id="menuOption1"> 
        <option value="1"> One </option>
        <option value="2"> Two </option>
        <option value="3"> Three </option>
        <option value="4"> Four </option>
        <option value="5"> Five </option>
    </select>
</span>

<span id="menuOption2-container">
    <select id="menuOption2"> 
        <option value="a"> Vowel a </option>
        <option value="e"> Vowel e </option>
        <option value="i"> Vowel i </option>
        <option value="o"> Vowel o </option>
        <option value="o"> Vowel u </option>
    </select>
</span>