how to change the css of default select2 placeholder color?

If I understand what you want correctly, you probably want to use this selector.

Original CSS which make the placeholder gray

.select2-default {
  color: #f00 !important;
}

Change your preferred placeholder color

.select2-default {
  color: #f00 !important;
}

Specific placeholder color (using id)

#s2id_<elementid> .select2-default {
  color: #f00 !important;
}

Replace with the original input or select id

In this case

#s2id_leadadd_mode_of_enq .select2-default {
  color: #f00 !important;
}

Also, another note for placeholder to work, you have to add an empty <option></option> or else the first option will be automatically selected, but not the placeholder.

Like so

<select id="leadadd_mode_of_enq" name="leadadd_mode_of_enq" class="select2 req_place" data-select-search="true" placeholder="Mode of enquiry">
  <option></option>
  <option value="1">Opt1</option>
  <option value="2">Opt2</option>
</select>

See Demo

the answers given are old and you can now use the css class

.select2-selection__placeholder {
    color: #FF0000;
}