I got my green mums by breeding special purple mums from special yellow mums that come from the combination of red and yellow mums. There's no way to tell which ones are special or not. I just kept only red and yellow mums together until they produced a purple, then I cloned the purple. Then the two purples eventually made a green mum. But they're rare, and it takes a while.
Based on what I've read about hybrids in this game, the only thing that could go wrong, other than bad luck making it take a really long time, would be if you had any white mums mixed in there. White mums will make non-special purple mums that will never make green mums. So as long as you have your mums isolated and never have any white ones in there, you can't help but get green ones eventually as long as you keep all the new yellow ones that spawn together until they make purples.
If you've done everything correctly so far, you're probably just having bad luck. Green mums are rare, and it can take a long time to get lucky enough for them to spawn from your special purple ones.
They have to be special purple mums, not just purple mums you breed from white + white. If they are special, it's just bad luck. Get a few friends to water them to increase the chances of them breeding.
I highly recommend bookmarking this thread, as it gets lost very quickly! Also, if you have any questions, feel free to reply and tag me (@Khaelis) in your post, or send me a DM/Conversation. I'll respond as soon as I am able. Hello, and welcome to my guide on hybrids for Animal Crossing...