Wordpress - Set Default Admin Colour For All Users

You can set (in terms of force) a default color within functions.php like this:

add_filter( 'get_user_option_admin_color', 'update_user_option_admin_color', 5 );

function update_user_option_admin_color( $color_scheme ) {
    $color_scheme = 'light';

    return $color_scheme;
}

Update: The following color schemes are available per default at WP 3.8

• fresh
• light
• blue
• coffee
• ectoplasm
• midnight
• ocean
• sunrise

Bonus (found on wpmudev): Disable the Admin Color Scheme Options to make sure that users can not switch back to another color:

remove_action( 'admin_color_scheme_picker', 'admin_color_scheme_picker' );

Update 2: As Rarst pointed out the filter above will force a specific color scheme instead of setting a changeable default. The solution to this is to run an action only once (e.g. on user setup/registration) so after that the user can decide and change the color on his own:

add_action( 'user_register', 'myplugin_registration_save', 10, 1 );
function myplugin_registration_save( $user_id ) {

       update_user_meta($user_id, 'admin_color', 'light');

}

Update 3: Okay, so one more try :)

The idea is to add extra user meta data (see custom_admin_color_scheme) as soon as the user updates the profile; as long as the field is not set to true we'll change the default admin color scheme to a color scheme of our choice:

// add custom user meta data
add_action('personal_options_update', 'save_custom_admin_color_optios');
function save_custom_admin_color_optios( $user_id ) {

    update_user_meta($user_id, 'custom_admin_color_scheme', true);

}

// change default color scheme if not customized
$customized_color_scheme = get_user_option( 'custom_admin_color_scheme', get_current_user_id() );
if ( empty($customized_color_scheme) ) {

    update_user_meta(get_current_user_id(), 'admin_color', 'light');

}

Update 4: Finally there is also a very nice plugin on wordpress.org to handle default admin color schemes easily: Default Admin Color Scheme