Nicolas, why do you want to change subtype? just assign metadata as you already done with favcolour.

Yeah Kevin that was my understanding as well I just thought to ask if someone had a workaround. The metadata value is what i use right now  I just wanted to use the subtype so i could utilize  list_entities and not  list_entities_from_metadata which sometimes works strangely for me ;)

-Edit: Ok from ytour answers it seems i shouldn't really bother with the subtype :) thank you very much all for the quick responses

I'm solving this with a SQL query:

$dbprefix = elgg_get_config('dbprefix');
$subtype_id = add_subtype('object', 'desired_subtype');
update_data("UPDATE {$dbprefix}entities
    set subtype='$subtype_id' WHERE guid=$entity->guid");

// If memcache is available then delete this entry from the cache static $newentity_cache;
if ((!$newentity_cache) && (is_memcache_available())) {
    $newentity_cache = new ElggMemcache('new_entity_cache');
}
if ($newentity_cache) {
    $newentity_cache->delete($guid);
}

Maybe is helpful for somebody.

Thanks for sharing this. Hope to see further uses of this.