Kayn passive is SO Confusing!!!

I played Kayn a weeek or 2 ago and played him enough to get both summoner icons, but today when I was playing him the passive wasn't working out how I thought it was supposed to.

Blue Kayn = Shadow Assassin Red Kayn = Darkin

Steps of what happened:

1. Meter fills up and gives me Red Kayn, didn't back so there were 2 min on Blue Kayn
2. Back to base, then transform into Red Kayn.
3. Could not transform into Blue Kayn.

I thought you could transform into blue kayn after I transformed into Red Kayn when the timer was up, or do I have to get more kills on ranged Champions? I'm assuming that the 4 minute timer is so if you got the wrong form you could still have a chance to transform into it.