1 | Red
2 | Blue
3 | Green

/**
 * Implements hook_form_FORM_ID_alter().
 */
function MODULE_NAME_form_YOUR_FORM_ID_alter(&$form, &$form_state) {

  // Replace the next 5 lines with your logic to set the array of values
  $options = array(
    1 => 'Red',
    2 => 'Blue',
    3 => 'Green',
  );

  $form['FORM_ELEMENT_ID']['#type'] = 'select';
  $form['FORM_ELEMENT_ID']['#options'] = $options;
}