Depending on whether your koi is (Gm/Gm S/S) or (Gm/g S/S) and if it crosses with a sunset (g/g S/S). If you koi are (Gm/Gm S/S) they're offspring will still be 100% koi after being crossed with the sunsets, because they will become (Gm/g S/S). But, if they're (Gm/g S/S) you will get a 50/50 mix of koi and sunsets. And unless you can contact where you bought them and ask the breeder for their genetics, you won't know which your kois are. Either way you won't loose the koi trait.