Enemy has yasuo brand morg cait rengar. which form do i go as a kayn player

Definitely blue, yasuo can get sort of beefy with and potentially same with rengar, so A: theres no enemy tanks to heal off of with Red Kayn R, and against a non-tank team blue kayns burst will be more valuable than red kayn's sustain, also arguably that enemy comp could burst a red kayn through his sustain.

Better to go for the 1 shots.

Depends on your team and what you need, but given theyre all squishy and have morg for reds CC, blue would get more benefit off the enemy alone.

And red cant exactly stop rengar, would likely be more useful against yas but thats it.

Unless your team can easily handle them and just need a beefy frontliner, blue would be a better pick for you alone.

This depends a lot on who is on your team as well and if you are ahead compared to rengar or any of the other champs. I would say assassin as cait is vulnerable despite her range. Ur mobile enough to get on top of her and one shot her, but then it becomes a game of who can one shot who first with rengar vs kayn. Another benefit of blue kayn would be that none of them are tank so u kinda just murder them all as long as ur not behind. However, if your team needs a tank/bruiser red kayn is fine as well because u can peel back on ur carries against the rengar yasuo. It really depends on a lot of things..

Only go blue if you get ahead.

You'll never 1 shot Yasuo or Rengar late game.

If Morgana has a brain, Blue Kayn won't play the game.

If you fall behind then go Red. Red is still useful even when behind.

If you manage to snowball a bit in lane phase then consider blue.