How to enter a default value to custom meta fields in WordPress

I try to create a small plugin using a WordPress plugin named “Easy Theme Options” plugin.

For example, I tried to add an option:

$eto_custom_meta_fields = array(
array(
        'label' => 'Menu box shadow color',
        'desc'  => 'Menu box shadow color (Default: #ffffff)',
        'id'    => $prefix.'shadow',
        'type'  => 'colorpicker'
    ),
...

I’d like to add a default value. I tried to add the following line:

'default'   => 'fffff',

But it did not work. How can enter a default value to a custom meta field?

Thanks for your help in advance