Element validate handler to ensure a hexadecimal color value.
function image_effect_color_validate($element, &$form_state) {
if ($element['#value'] != '') {
$hex_value = preg_replace('/^#/', '', $element['#value']);
if (!preg_match('/^#[0-9A-F]{3}([0-9A-F]{3})?$/', $element['#value'])) {
form_error($element, t('!name must be a hexadecimal color value.', array(
'!name' => $element['#title'],
)));
}
}
}