_color_preset.scss

.grey {
    background-color: #CCC;
    color: black;
}

.yellow {
    background-color: yellow;
    border: 1px solid red;
}

.pink {
    background-color: deeppink;
    color: white;
    font-size: 120%;

    h2 {
        text-transform: uppercase;
    }
}

color-preset.inc

<?php 
/**
 * @file
 * Custom panels color presets.
 */

// Plugin registrieren
$plugin = array(
  'title' => t('Color preset'),
  'description' => t('Set a color preset.'),
  // Achtung: render pane wird im Methodenaufruf mit "theme_" prefixed!
  'render pane' => 'MYTHEME_color_preset_style_render_pane',
  'pane settings form' => 'MYTHEME_color_preset_settings_form',
  'category' => t('MYTHEME'),
);

/**
 * Renders the pane content.
 */
function theme_MYTHEME_color_preset_style_render_pane($content) {
  if (!empty($content)) {
    return theme('panels_pane', $content);
  }
}

/**
 * Color preset settings form.
 */
function MYTHEME_color_preset_settings_form($style_settings) {
  $form = array();
  $form['preset_color'] = array(
    '#type' => 'select',
    '#title' => t('Choose color preset'),
    '#default_value' => (isset($style_settings['preset_color'])) ? $style_settings['preset_color'] : FALSE,
    '#options' => theme_MYTHEME_color_preset_colors(),
  );

  return $form;
}

/**
 * Available color presets.
 * @return array
 */
function theme_MYTHEME_color_preset_colors() {
  return array(
    'grey' => t('Grey'),
    'yellow' => t('Yellow'),
    'pink' => t('Pink')
    // etc.
  );
}

MYTHEME.info

; Register Style Plugin directory
plugins[panels][styles] = panels/styles

panels-pane.preprocess.inc

<?php 
/**
 * @file
 * Preprocess panels color presets.
 */

/**
 * Implements hook_preprocess_panels_pane().
 */
function MYTHEME_preprocess_panels_pane(&$variables) {

  // Prüfen ob unser Style Plugin aktiv ist.
  if(isset($variables['pane']->style['style'])) {
    if($variables['pane']->style['style'] === 'color_presets') {
      // Setzt die in o.g. theme_MYTHEME_preset_colors definierte Farbe als CSS-Klasse
      $variables['attributes_array']['class'][] = $variables['pane']->style['settings']['preset_color'];
    }
  }

  return $variables;
}