There's a good description already posted in the Guides of when it's best to use the mutation liquid - and seedling isn't it, I'm afraid. It does always change something - the foliage or the flower, but when the plant isn't mature, you can't really tell what's changed. If you wait until the plant is mature, you can use it then and definitely see a difference! It is costly, and to me more or less a last resort, but I got lucky the first time I used it. Haven't used it since, though, and am afraid I'll grab the wrong bottle and mutate something I really don't want to!

Good luck!