I think there is a way to know for sure the rank it will put you in, for those skeptical since some players are reporting the above not being always the case.
Before joining the event you can check the reward by clicking the reward button.
This will show you all three tiers and you can check the rewards for each one.
In that very screen page(bronze, silver, gold tier) you will notice that one is outlined.
It is current tier you should be in for the game.
Again, I am not a dev, but logic tells me this confirmation by the game should be enough to guarantee the tier you will be put into when joining.
Unfortunatly, this is not true. I was silver tier, going to gold for this NoP event. I waited to join until I was gold tier. I got to gold in red, and saw gold rewards light up when I got to the rewards page. After I joined, I looked at the rewards for the event and it displayed the silver tier rewards… I think it is decided when the event becomes active, instead of when you join.
I’m pretty sure it’s based on which planeswalker you start your first match with. I only just made it to Silver in Green before this last event, and was in Bronze with the rest, so I led off my first match with Green and found myself in a Silver-tier set.
Nah, I’m gold with green and red, but started the last event with white, and got gold rewards. It should be putting you into the highest tier you qualify in any color for. If it’s not, I suspect it’s a bug.